1. Every extension you add seems to decrease performance by 30-50% (rough numbers) [1]. So live with a bare essential set of extensions. For me that is Firebug and FlashBlock.
2. Keeping very many bookmarks or history items seems to destroy performance, particularly with the "awesome bar." I disable history completely and have it clear everything else on browser close.
Before I settled on this FF usage technique, FF would degrade in performance over the course of months as it built up data in the profile. Eventually it would become unusable [2] until I deleted the profile and started fresh. Now I just prevent it from building up any data.
[1] As much as I used to like this extension, my experience has been that ForecastFox in particular hammers browser performance.
[2] It would take seconds, sometimes 10 or more, to open a new tab to about:blank. With this technique though, I currently have ~ 25 tabs open, and new tab creation is instantaneous.