Your bookmark database (places.sqlite) may be locked or damaged. Try rebuilding it:
- Export a backup of your bookmarks (just in case)
- Close Firefox
- Go to your FF profile folder:
http://kb.mozillazine.org/Profile_folder_-_Firefox- Delete *all* files the names of which start with "places.sqlite"
- Start Firefox - the database should be rebuilt
- Try saving a bookmark and see if it works now
For more, check here:
http://kb.mozillazine.org/Locked_or_damaged_places.sqliteIf this won't work, your complete profile may be corrupted. Make a new one, import your bookmarks, try saving a new bookmark.
http://kb.mozillazine.org/Profile_Manager#Creating_a_new_profileps:
This is actually already a second question. Your original question was whether there are limits.