note: not all extensions will work right away. Bookmark Synchronizer doesn't, which is a bummer, as it's the one I use the most.
I found some info to get Bookmarks Synchonizer to work with 1.5
- Download v1.0.1
- Open the .xpi file with WinZip, WinRAR, some utility that can open/write to a zip archive
you'll find the em:maxVersion set at 1.0 --> Change that to 1.6 and update the .xpi file
- Install the extension and you should (hopefully!!!) be good to go.