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.