You can find any number of posts which say 'Go to about:config...'
This isn't one of them. Seriously. There are loads of posts of them that say this and not one that says the bleeding obvious.
Go to the addons search for Firefox, and install Nightly Tester Tools (Not the lite version).
Go to the addons window. Look in the bottom right and you'll see a new button saying 'Override all compatibility'. Click it, and restart Firefox once more.
Done. Your addons now work. If you have incompatible themes, go to the themes selector and repeat.
When you install an incompatible addon, it'll tell you, but now you can tell it to override and install anyway.
HOW DAMN SIMPLE IS THAT?
Ahem. Sorry for the amount of caps there.
Enjoy your quick and dirty way to get all addons working in Firefox 3.5, and remember - this trick will always be the same, in any version you can add the Nightly Tester Tools to.