In my experience, Firefox gets slower and slower the longer it's been open and with many tabs open. I believe it to be related to GC. Unfortunately, there aren't any hints (like per-tab activity/memory/allocation/timer readouts) as to which tabs are contributing the most to the problem. Sometimes I'll guess the miscreants and closing enough of them helps, but more often things only improve after a restart (essentially unloading all tabs until revisited).
It does seem add-ons are implicated (and perhaps especially Firebug, even if Firebug is not activated in any tab). NoScript may also be implicated; especially with regard to pages that have some but not all script sources enabled.
(UPDATE:) Per <http://support.mozilla.org/en-US/kb/reset-firefox-easily-fix..., it appears 'about:support' is the right place. I suspect this would help, but the problem would recur as soon as I restore my core Add-ons and usual tabs.
In my experience, Firefox gets slower and slower the longer it's been open and with many tabs open. I believe it to be related to GC. Unfortunately, there aren't any hints (like per-tab activity/memory/allocation/timer readouts) as to which tabs are contributing the most to the problem. Sometimes I'll guess the miscreants and closing enough of them helps, but more often things only improve after a restart (essentially unloading all tabs until revisited).
It does seem add-ons are implicated (and perhaps especially Firebug, even if Firebug is not activated in any tab). NoScript may also be implicated; especially with regard to pages that have some but not all script sources enabled.