Glad someone started this thread.

I'm running version 3.6.17 and I like it.
I don't have any Ipods or phones...etc. that I use with Firefox.
I don't use more than 3-5 tabs....ever.

From what I'm reading, I don't think I'll change to 4.0
Can't see the benefits, for me anyway.
No crashes now, ever, and seems fast enough for me.

If it's not broke....don't fix it.

Upgraded to 4, no issues with add-ons, but I don't use many these days other than Tabmix Plus. Have had a couple of temporary lock ups, but no crashes. I do the majority of my browsing on my netbook which is significantly faster on 4. Can't tell much difference on my desktop, but I don't use that for browsing much to tell.
Always good to back up your bookmarks every so often regardless. TabMix Plus options too if you use it.

Rule #1 of PC security, keep your software up to date unless you have a very compelling reason not to.