How do I use the Firebug extension to alert me of a website sending ajax calls to my web browser
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I'm trying to purchase items that are out of stock on a website. I don't want to have to constantly check the page for updates and I was told firebug will monitor this for me. I do know that the website uses Ajax messages to update their current inventory. I've found the line of Html that is changed but that is the extent of my web development knowledge. 

With the information I already know about the web page how do I get Firebug to alert me of the change?

I really appreciate the help, I tried searching around but couldn't seem to find an applicable answer for my situation.