Well, I recently did this and was surprised to see most of my Firefox extensions of 2.0 not working in the new Firefox 3.0. The only exception was DownloadThemAll (or DTa as it is popularly called). As usual, I have a trick on my sleave to get across with this problem. Though this method is not fool proof - but in my case it worked without a blip.
Here are the steps you need to follow:
1. Download the .xpi file of the extension (You can use Internet Explorer for this)
2. Rename the downloaded file .zip from the .xpi
3. Now open the new zip file using WinZip / WinRar or any other file compression software.
4. Inside the zip - Open the install.rdf in edit mode
5. Locate and Replace 2.0.0.* by 3.* in Mozilla Firefox section.
6. Save the .rdf file and rename zip archive back to .xpi
7. Now Install the extension.(Double click the .xpi file and open in Firefox)
It works as expected. All the very Best!!