Are you possibly using Sync to sync bookmarks? If you use Sync then best is to disconnect Sync temporarily during troubleshooting. You can check for problems with the <b>places.sqlite</b> (bookmarks and history) and <b>favicons.sqlite</b> files in the Firefox profile folder. *use "Verify Integrity" button on the "Help -> Troubleshooting Information" (about:support) page<br>in case errors are reported close and restart Firefox and retry See also: *http://kb.mozillazine.org/Locked_or_damaged_places.sqlite *https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file If "Verify Integrity" button on the "Help -> Troubleshooting Information" (about:support) page cannot repair places.sqlite then remove all places.sqlite files in the Firefox profile folder. Firefox will rebuild places.sqlite and restore the bookmarks from a recent JSON backup in the bookmarkbackups folder. *keep a backup copy of places.sqlite in case a new places.sqlite database has to be created *http://kb.mozillazine.org/Unable_to_process_the_backup_file_-_Firefox You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:preferences</b> page. *Help -> Troubleshooting Information -> Profile Directory:<br>Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder *http://kb.mozillazine.org/Profile_folder_-_Firefox

Thank you it seems that the issue was in doing large scale reorganising of my bookmarks with sync on. I simply turned off sync, & reordered/deleted duplicates & then turned sync on again. So far all is well.