There have been some performance issues with the Sync(Basic) servers yesterday and today.
http://status.mozilla.com/11627/149178/Sync-%5BBasic%5D

I'll wait a few days and then report back

Try to disconnect from Sync (Unlink This Device) on the desktop computer, quit Firefox, delete places.sqlite, restart Firefox, and re-set up Sync.

You can check for problems with the places.sqlite database file in the Firefox Profile Folder.

• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• https://support.mozilla.org/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file

Waiting did the trick, it works now. Strange that the error messages and server status were not more expressive

Since yesterday, that's when I tried to use Firefox Sync since a long time, the service seems to be not available. Or at least very unresponsive.

Is that true (I cannot identify the Sync Server on above linked the Status page), or is that an issue with my machine?

I have the same issue. I tryed to create a new profile and even a new sync account - but nothing helped. the sync didn't working.

I can publish the log from about:sync-log

several days now synch failed and "server maintenance is underway" is blamed. this happens all day. Starting yesterday my Bookmarks menu at the top of the Mac screen has been freezing up. Use this all the time and has never happened before.

This latest version of Firefox 21.0 has been a bummer. Still got the restore bug.