Whenever Firefox memory use has ballooned to over 1GB.

Which is pretty much daily.

After a while when there are many tabs opened, I find it easier to start anew by closing the browser (in fact I kill the task).

There is also a shared computer in my home that is used by my wife and my kids; there, browsers are constantly opened and closed since for some reason, most non-tech people (that I know -- and that includes my wife) close and reopen the browser to start a new browsing session.

I use Firefox mostly with Selenium, to run my automated tests, so I do it numerous times each day.