I am using Firefox 3.6.1 most of the time (on various computers running, in order of usage frequency, Windows Vista Home Premium, Windows XP, Windows XP Pro, with respectively 2GB Ram, 2GB Ram, 1.5GB Ram with usually a Verizon Fios connection but at times Verizon Evdo Broadband mobile connection.) Sadly, over the past couple/few years, it went from a sleek browser of competitive speed to one of excessive sluggishness and bloat. I do not believe this to be caused by add-ons, at least in my case, as I do not use very many. I have experimented with varying numbers and combinations of add-ons with quite similar results. I've even tried no add-ons at all but saw no discernable speed differences.
From time to time I resort to using IE or Chrome when, for example, I need to do something very quickly, especially when Firefox has many tabs open and is choking on the simplest of actions - like closing a tab or two.
Yes I am a fairly hefty tab user and frequently have 35, 50 or more tabs open at once. And yes I do have good reason to want to use that many tabs or more (has to do with a type of searching and research I frequently do.) This, a big bonus in Firefox, may also be a factor in it's slowness. I don't know - but I have seen Firefox crawling with as few as 12-16 tabs open at once.
As to what add-ons I consider "must haves", these would be Scrapbook, TabMixPlus , and Copy All Urls. I also very much appreciate the built in Session manager and the TabMixPlus version. If Chrome or IE were to have these few features available, as built ins or add-ons, I would likely be more than willing to switch as I am so tired of constant wait time. Sometimes I feel I am back in the days of dial-up connections over a 56k modem. Thats how bad it is.
Oh and also want to mention the crash factor - I've not seen it be that bad with 3.6 but all 3.5 versions were unbearably crashable. What happened to this browser over time? It began so wonderfully!