A possible cause is a problem with the file places.sqlite that stores the bookmarks and the history.

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

There may now not be much that can be done. Immediately find your current profile the easiest way of doing that is using

• Help -> Troubleshooting -> Profile [open containing folder]

Copy the whole profile folder with all sub files and folders and put it somewhere safe, outside Firefox program or Profile paths (Windows desktop is a suitable location)

Once you have done that it is also worth exporting any bookmarks you have as a backup, do it as firefox backup, and HTML, it is just possible they may contain usefull info in an easily accessble form.

If the bookmarks were somehow corrupted, and then you have overwritten current bookmarks with corrupt backups there may not be much that can be done,but if you copy the profile as I mentioned above at least we still have the files and can see if anything is rescueable. There is also a possibility the files are okand the fault is with firefox,but that may be less likely.

• see also Recover lost or missing Bookmarks
• http://kb.mozillazine.org/Bookmarks_history_and_toolbar_buttons_not_working_-_Firefox
• and the oddly named Fix "The bookmarks and history system will not be functional" error message

Thanks for your help, guys. It turned out a couple of places.sqlite.corrupt files existed and just by renaming one of them could be used again. Turned out to be a full year old, so I've lost all my recent bookmarks, but better than nothing.

Thanks for your advice on backups too, John.

Best regards, Steve