You can open about:sync-log via the location bar to see if there is a recent error log available. You can upload a recent log if the log is too big to put in a reply.

• https://pastebin.com/

Thanks - I'm not familiar with pastebin so hope I've done this right!

<script src="https://pastebin.com/embed_js/U6s4DvCx"></script>

You can see at line 115 and further that there is a problem with the bookmarks.

1544985168110   Sync.Engine.Bookmarks   WARN    Error creating record: TypeError: http:/// is not a valid URL.(resource://gre/modules/Bookmarks.jsm:2441:18) JS Stack trace: rowsToItemsArray/<@Bookmarks.jsm:2441:18
rowsToItemsArray@Bookmarks.jsm:2425:10
query@Bookmarks.jsm:1969:26
1544985168161   Sync.Engine.Bookmarks.BookmarkSyncUtils DEBUG   pushChanges: Processed change records: {"weak":0,"skipped":2722,"updated":0}

You can check in the Bookmarks Manager (Library) if you can locate a "http:///" bookmark.

You can check for problems with the places.sqlite database (bookmarks and history) in the Firefox profile folder.

• use "Verify Integrity" button on the "Help -> Troubleshooting Information" (about:support) page
  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
• https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

Thanks - there was a corrupted bookmark with "///" that wouldn't delete so I had to delete the sqlite files and it seems to have cleared up!