I have been working under assumption that the browser gets less snappy as more tabs are opened. This increases the chances of having an ill-behaved website in the background. An ill-behaved tab (or a couple of them) can in theory ruin scrolling, typing, clicking, etc in active tabs. However I do not have anything behind anecdotal evidence on this. There are bugs on specific websites in bugzilla, but it would be nice to get them mixed into a realistic set of tabs.
Would someone be willing to contribute a list of webpages they use often that cause Firefox to lag (maybe a session restore file?)? I am a low-tab person myself, so I can’t easily reproduce this. Please make sure that Firefox is slow with your list of tabs even when all addons are disabled, include a description of slowness encountered.