This might be this Firefox bug: https://bugzilla.mozilla.org/show_bug.cgi?id=263160 - related to GTK, it only happens with (say) 50 to 100 tabs or more. The main symptom is that IFRAMEs 'pop out' as windows, and eventually Firefox hangs or crashes.
This bug has been open from Firefox 1.0 to 3.5.x, so if any Firefox or GTK experts could take a look at it, I'm sure some heavy Firefox users would be thankful. Currently Firefox works much better on Windows than Linux when you have 100+ tabs, for this reason.