...Mozilla was still supporting 3.5? Daaaang!
Considering the differences between 3.5 and 3.6 (they are few: integrated support for Personas, plugins sandboxing since 3.6.4, a few CSS3 properties, and a faster JS engine), NOT updating is foolish; in fact, I would more see problems with users of 3.0 who refused the ugrade to 3.5/3.6.
The extension problem is a false problem: 3.5 extensions have, by now, gotten more than enough time to update for 3.6 support - so it's pretty much instantaneous, when you load 3.6 for the first time, a few of them will be upgraded and that's it. Except if you disabled support for extensions updating.
As for 4.0's UI being much different than 3.x: go to Firefox button, Toolbars > Menu Bar reverts a bunch, uncheck 'tabs on top' on that same menu, and there you have it: pretty much the same as before. There is an extension restoring the status bar, and at that point you go back to Firefox 3.0's interface; and then, there is a theme reverting Fx 4 to Fx 2/3's appearance!
So, not updating reverts to "using deprecated, dangerous, non-supported extensions on your main browser". Like running with scissors.