Open Firefox in the Firefox SafeMode by holding the Shift key when double-clicking on the Firefox desktop shortcut. Then open the Add-ons Manager > Extensions tab and disable the Xmarks extension. Then close Firefox.

When you open Firefox again see about getting help from Xmarks.
http://www.xmarks.com/about/help

The suggested solution is not very helpful. I depend on Xmarks and disabling it is unacceptable.

Download the latest version of xmarks and install it.

[http://download.xmarks.com/download/firefox Xmarks for Firefox Hope this will help :)

I have had this problem for months, if not by now, years. I think I have finally found a solution that works most of the time. Steps below.

1. Back up your bookmarks

2. Delete your bookmarks

3. Remove Xmarks from your add-ons

4. After FF restarts, exit the application

5. Go to ../.mozilla/firefox/*.default and remove the 2-3 files starting with xmarks*

(Windows users will find this, I think, in <username>/AppData/Local/Mozilla/... but you may have to use the command line to get there. Once in AppData in the command line you can type "explorer ." to open up a Windows Explorer session.)

6. Start FF

7. Use the extension manager to find and install Xmarks

8. I have to run the setup wizard manually, as I haven't found where it is getting it's Last Sync: date from, but it's persistent (and part of the problem, I believe)

9. In setup wizard I choose 'download from server/discard computer' or whatever it really says -- you should get the idea

10. After some disk grinding it seems to work (for awhile until it breaks again, which won't be long)

11. Remember that Xmarks used to be Foxmarks, as in for Firefox

12. If all else fails, at least you have the backup -- you did create a backup, right?

I did HunkirDowne's steps and also reset all the xmarks entries in about:config (that's where the last sync date and lots of other not-uninstalled data is). That seems to have worked so far.