Anyone using an old version - welcome to the hackers. And kiss goodbye to enjoying all the benefits of later versions, such as HTML 5 (inbuilt video and so on). Do people really think each release is a downgrade? Why not stick with version 1 then!
Chrome is interesting because it AUTOMATICALLY UPDATES ITSELF. So there's none of this "I refuse to upgrade" nonsense. People are always using the latest version, which has to be a good thing. (I remember in the past people still clinging to Netscape 4!)
I keep a lot of tabs open in FF (too many sometimes) and occasionally it'll get slow, but only after a lot of usage. One tip I've found is to clear the cache now and then. That speeds it up.