Click tools->addons, and see what addons/extensions you have in firefox2.
Then click the 'get more addons' link, to take you to the addons page for downloading new ones.
Find the pages for your current addons, and see what versions of firefox they're compatible with. If all of them work with firefox3 too, then you'll be fine.
If an extension you can't live without isn't yet working under firefox3, then don't bother changing unless the new features compel you to.