Get the portable firefox 3.5 edition.
It will run on your system as long as you close the older version (no need to uninstall it).
You can also use the FEBE and CLEO extensions to package your present extensions for a quick transfer. However, I have not tested if that works across a version upgrade. Some of them will of course not install at all.
Another easy way after you setup portable firefox 3.5 is,
If you create a collection of extensions on the firefox site,
you can just open the collections page from the 3.5 and scroll over the addon list.
All unsupported addons will be identified immediately.
This collection will also be usable for installing the entire list on this version or any other firefox installation you have elsewhere.
And, this will work for all future upgrades.