50%. Is this an average number? How many extensions really need more than an configuration change?
If you’re looking for your favorite extension, check out this.
I’ll miss the SwitchProxy Tool most.
Update: thanks to Jeff, I got almost all my extensions back using this
Bistro! 2.0 
Hi Alexis, also check out the homepages for updates , I found updates that were not reported by the UI, and ProxyButton works well see http://proxybutton.mozdev.org/ :-) I use TabMixPlus, IE Tab & Flashblock as utilities. Matt
(with ID)
You should try out the Nightly Tester Tools (http://users.blueprintit.co.uk/~dave/web/firefox/nightly). It allows you to mark any extension as compatible even if it doesn’t think that it is. I used it to mark most of my extensions as compatible and almost everyone of them worked fine.
I think you should have a look at “Mobile Net Switch” (http://www.mobilenetswitch.com/)
I had a couple of extensions that I wanted to keep, but that were not compatible with 2.0. I downloaded the xpi files instead of installing them, opened the xpi with Total Commander (any zip tool should do the trick) and edited the file install.rdf. There are two entries for FF and TB, named <em:maxVersion> where I changed the value to 2.0+. After putting the edited file back in the xpi, it is just a matter of dragging the xpi into an open FF window.
This has worked for me for those extensions that I wanted to keep, but in case you have an extension that really is not compatible, I don’t know what the outcome would be.
Thanks all. Jeff, this is a nice tool!