title: Slow SVG/JavaScript response only in Firefox: is it me or is it Mozilla?
body: <p>I've written a drag-and-drop SVG page that has to monitor mousemove events and update the page (stretching some connecting lines between rectangles that are being dragged around and drawing lines when I drag one rectangle across another, changing the connection points). </p>
  
  <p>I understand that monitoring mousemove means responding to a very large number of events. Nonetheless, the responsiveness is blazingly fast in Chrome and Opera, and plenty good enough in Safari and IE, but it's unacceptably jerky in Firefox (current versions of all browsers under Windows 7 and all except IE under MacOS Mavericks). </p>
  
  <p>Disabling add-ons has no effect. Profiling doesn't show any obvious bottlenecks, and the Chrome and Firefox profiles look similar. That page is at <a href="http://repertorium.obdurodon.org/dev/test.xhtml" rel="nofollow">http://repertorium.obdurodon.org/dev/test.xhtml</a>, if anyone cares to look, but the question is more about whether this </p>
  
  <ol>
  <li><p>A known Firefox problem with SVG DOM manipulation that has no known work-around until Mozilla updates the JavaScript engine; </p></li>
  <li><p>a known Firefox problem with a known "best practice" workaround; or </p></li>
  <li><p>likely to be developer error. Thanks for any guidance.</p></li>
  </ol>
  
  <p>[Edit: added JavaScript in response to suggestion from @JosephtheDreamer. Trying to add even a reduced HTML/SVG file that would illustrate the problem wound up exceeding the StackOverflow 30,000-character length limit]</p>
  
  <pre><code>"use strict";
  /*
   * Synopsis: Drag and drop support for plectograms
   * Developer: David J. Birnbaum, djbpitt@gmail.com http://www.obdurodon.org
   * Project: http://repertorium.obdurodon.org
   * Date: First version 2014-05-30
   * Last revised: 2014-06-22
   *
   * To do:
   * js:
   *   Wrap initialization in self-executing anonymous function (SEAF) to
   *     bypass addressing the window 'load' event explicitly:
   *     http://code.tutsplus.com/tutorials/key-principles-of-maintainable-javascript--net-25536
   * xhtml/svg:
   *   Adjust hierarchy of XQuery output to match mockup markup
   *
   * drag and drop based on http://dl.dropboxusercontent.com/u/169269/group_drag.svg
   * see also http://www.codedread.com/dragtest2.svg
   */
  window.addEventListener('load', plectogram_init, false);
  var djb = function () {
      return {
          adjustLinesAndColumns: function (how) {
              // only values should be 'fade' and 'restore'
              var lines = document.getElementsByTagName('line');
              var lineColor = how == 'fade' ? 'lightgray' : 'black';
              var columnOpacity = how == 'fade' ? .5 : 1;
              for (var i = 0; i &lt; lines.length; i++) {
                  lines[i].style.stroke = lineColor;
              }
              var columns = document.getElementsByClassName('draggable');
              for (i = 0; i &lt; djb.columnCount; i++) {
                  if (columns[i] !== djb.newG) {
                      columns[i].style.opacity = columnOpacity;
                  }
              }
          },
          assignEventListeners: function (g) {
              // other listeners are on window, so that they can be trapped when the mouse races ahead
              g.getElementsByTagName('image')[0].addEventListener('mousedown', djb.startMove, false);
              window.addEventListener('mousemove', djb.moveIt, false);
              window.addEventListener('mouseup', djb.endMove, false);
          },
          buildDict: function (g) {
              // attach a dictionary to each &lt;g&gt; with text as key and vertical position as value
              // also adds .getXPos() and .setXPos() methods to &lt;g&gt; object
              g.contents = {};
              var columnCells = g.getElementsByTagName('g');
              for (var i = 0; i &lt; columnCells.length; i++) {
                  var cellText = columnCells[i].getElementsByTagName('text')[0].textContent;
                  var cellYPos = columnCells[i].getElementsByTagName('rect')[0].getAttribute('y');
                  g.contents[cellText] = cellYPos;
              }
              g.getXPos = function () {
                  return parseInt(this.getAttribute('transform').slice(10, -1));
              };
              g.setXPos = function (X) {
                  this.setAttribute('transform', 'translate(' + X + ')');
              };
          },
          createNewG: function (image) {
              djb.newG = image.parentNode.cloneNode(true);
              djb.buildDict(djb.newG);
              djb.newG.oldX = djb.newG.getXPos();
              image.parentNode.parentNode.appendChild(djb.newG);
              image.parentNode.parentNode.removeChild(image.parentNode);
          },
          drawLines: function () {
              var columns, key, x1, y1, x2, y2, linesG, newLine, i;
              columns = djb.htmlToArray(document.getElementsByClassName('draggable'));
              columns.sort(function (a, b) {
                  var result = a.getXPos() - b.getXPos();
                  return (result);
              }); // numerical sort by Xpos of column
              linesG = document.getElementById('lines');
              for (i = 1; i &lt; djb.columnCount; i++) {
                  x1 = columns[i].getXPos();
                  x2 = columns[i - 1].getXPos() + djb.columnWidth;
                  for (key in columns[i].contents) {
                      if (columns[i].contents.hasOwnProperty(key)) {
                          if (columns[i - 1].contents.hasOwnProperty(key)) {
                              y1 = parseInt(columns[i].contents[key]) + djb.columnMidHeight;
                              y2 = parseInt(columns[i - 1].contents[key]) + djb.columnMidHeight;
                              newLine = document.createElementNS('http://www.w3.org/2000/svg', 'line');
                              newLine.setAttribute('x1', x1);
                              newLine.setAttribute('y1', y1);
                              newLine.setAttribute('x2', x2);
                              newLine.setAttribute('y2', y2);
                              newLine.setAttribute('stroke', 'darkgray');
                              newLine.setAttribute('stroke-width', '2');
                              linesG.appendChild(newLine);
                          }
                      }
                  }
              }
              djb.newG.oldX = djb.newG.getXPos();
          },
          endMove: function (evt) {
              evt.preventDefault();
              var i;
              window.removeEventListener('mousemove', djb.moveIt, false);
              window.removeEventListener('mouseup', djb.endMove, false);
              var columns = document.getElementsByClassName('draggable');
              var allNewColumnPositions = [];
              for (i = 0; i &lt; djb.columnCount; i++) {
                  allNewColumnPositions.push(columns[i].getXPos());
              }
              // numerical array sorting at http://www.w3schools.com/jsref/jsref_sort.asp
              for (i = 0; i &lt; djb.columnCount; i++) {
                  if (allNewColumnPositions.indexOf(djb.initialColumnPositions[i]) == -1) {
                      djb.newG.setXPos(djb.initialColumnPositions[i]);
                  }
              }
              djb.eraseLines();
              djb.drawLines();
              djb.adjustLinesAndColumns('restore');
              djb.newG = null;
          },
          eraseLines: function () {
              // someday this will be easier: http://red-team-design.com/removing-an-element-with-plain-javascript-remove-method/
              // must work backwards: http://stackoverflow.com/questions/1457544/javascript-loop-only-applying-to-every-other-element
              var lines = document.getElementsByTagName('line');
              for (var i = lines.length - 1; i &gt;= 0; i--) {
                  lines[i].parentNode.removeChild(lines[i]);
              }
          },
          htmlToArray: function (htmlCollection) {
              return Array.prototype.slice.call(htmlCollection);
          },
          moveIt: function (evt) {
              evt.preventDefault();
              var oldObjectX = djb.newG.getXPos();
              var newObjectX = oldObjectX + evt.clientX - djb.mouseStartX;
              djb.newG.setXPos(newObjectX);
              djb.stretchLines();
              djb.mouseStartX = evt.clientX;
              if (newObjectX &lt; djb.objectX - djb.spacing &amp;&amp; newObjectX &gt; '0') {
                  djb.swapColumns('left');
              } else if (newObjectX &gt; djb.objectX + djb.spacing &amp;&amp; djb.objectX &lt; djb.farRight) {
                  djb.swapColumns('right');
              }
          },
          setXPos: function (g, Xpos) {
              g.setAttribute('transform', 'translate(' + Xpos + ')');
          },
          startMove: function (evt) {
              // global variable, used by djb.moveIt()
              djb.mouseStartX = evt.clientX;
              /*
               * clone clicked &lt;g&gt; into djb.newG and attach event listeners
               * svg has no concept of z height, so replace with clone at end of element sequence to avoid
               *   losing focus by becoming hidden by later elements
               */
              djb.createNewG(this); //image element that was clicked is the hook
              // djb.objectX is global, records starting position of drag
              djb.objectX = djb.newG.getXPos();
              djb.assignEventListeners(djb.newG);
              djb.adjustLinesAndColumns('fade');
          },
          stretchLines: function () {
              var newGX = djb.newG.getXPos();
              var lines = document.getElementsByTagName('line');
              for (var i = 0; i &lt; lines.length; i++) {
                  var x1 = lines[i].getAttribute('x1');
                  var x2 = lines[i].getAttribute('x2');
                  if (x1 == djb.objectX + djb.spacing || x1 == djb.newG.oldX + djb.spacing) {
                      lines[i].setAttribute('x2', newGX + djb.columnWidth);
                  } else if (x2 == djb.objectX - djb.spacing + djb.columnWidth || x2 == (djb.newG.oldX - djb.spacing + djb.columnWidth)) {
                      lines[i].setAttribute('x1', newGX);
                  }
              }
          },
          swapColumns: function (side) {
              var columns, neighbor;
              columns = djb.htmlToArray(document.getElementsByClassName('draggable'));
              columns.sort(function (a, b) {
                  var result = a.getXPos() - b.getXPos();
                  return result;
              }); // numerical sort by Xpos of column
              neighbor = columns[djb.objectX / djb.spacing - 1];
              if (side == 'left') {
                  djb.objectX = djb.objectX - djb.spacing;
                  neighbor.setXPos(neighbor.getXPos() + djb.spacing);
              } else {
                  djb.objectX = djb.objectX + djb.spacing;
                  neighbor.setXPos(neighbor.getXPos() - djb.spacing);
              }
              djb.eraseLines();
              djb.drawLines();
          },
          columnsHTML: document.getElementsByClassName('draggable'),
          dummy: null
      }
  }();
  function plectogram_init() {
      var images, i;
      djb.columns = djb.htmlToArray(djb.columnsHTML);
      for (i = 0; i &lt; djb.columns.length; i++) {
          djb.buildDict(djb.columns[i]);
      }
      djb.columnCount = djb.columns.length;
      djb.columnHeight = parseInt(djb.columns[0].getElementsByTagName('g')[0].getElementsByTagName('rect')[0].getAttribute('height'));
      djb.columnMidHeight = djb.columnHeight / 2;
      djb.columnWidth = parseInt(djb.columns[0].getElementsByTagName('g')[0].getElementsByTagName('rect')[0].getAttribute('width'));
      djb.initialColumnPositions = [];
      for (i = 0; i &lt; djb.columns.length; i++) {
          djb.initialColumnPositions.push(djb.columns[i].getXPos());
      }
      djb.spacing = djb.initialColumnPositions[1] - djb.initialColumnPositions[0];
      djb.farRight = djb.initialColumnPositions[djb.initialColumnPositions.length - 1];
      djb.newG = null;
      djb.mouseStartX = null;
      images = document.getElementsByTagName('image');
      for (i = 0; i &lt; images.length; i++) {
          images[i].addEventListener('mousedown', djb.startMove, false);
      }
  }
  </code></pre>