I mean, I don't want want to be the "640K oughtta be enough for everybody" person here, yet somehow I can't accept that it should take a quarter gigabyte to render a "hello world" HTML document. Every morning when I get into work, the first thing I have to do is click a button that I installed into Firefox to restart it so that my machine (4Gb RAM) becomes responsive. (That this important button even has to be a third-party add-on indicates serious reality denial.)

This sounds like atypical behaviour. One option is to try resetting Firefox, which gives you a new profile while keeping your history, bookmarks, etc: https://support.mozilla.org/en-US/kb/reset-firefox-easily-fi.... It can fix a lot of weird problems.

If that still doesn't help, I'd be interested to see what about:memory says. Instructions are at the top of https://developer.mozilla.org/en-US/docs/Mozilla/Performance.... Please file a bug in Bugzilla or email me. Thanks.