system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do browsers continually use up more memory the longer they are open?

Edit: Thanks For all the helpful info guys.
gpt
Because the more pages you browse, the more data it has to cache.

Try this:

Keep opening new websites on the same tab. Open 15 different sites.

Then keep on clicking the back button. 

You'll notice that the last 10 pages (I think) will load up instantly from where you left them. It is because they are still present in RAM, and this is the major contributor of memory usage.

There is a way to reduce the no. of pages to keep in memory per tab in about:config. A google search will give you instructions.