There is only a limit for the payload used for synced tabs and a date/time range for the history. I don't think there is a limit for the bookmarks, but bookmarks are synced in chunks of a few hundred items (200), so it takes some time to sync a large amount of bookmarks.

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

• https://pastebin.com/

You can temporarily toggle this pref to true to get a Sync log in case of success to check how Sync performs.

• services.sync.log.appender.file.logOnSuccess = true

Do not forget to reset services.sync.log.appender.file.logOnSuccess after you are done testing.

You can open the about:config page via the location/address bar. You can accept the warning and click "I accept the risk!" to continue.

• http://kb.mozillazine.org/about:config

Thanks for the comment.

The latest entry from about:sync-log (error log)

https://pastebin.com/vqUk3SGQ

There seem to be errors with the bookmarks and you also seem to quite a few of them as I see a mention of over 3000 blocks. If I look at the log then I see a lot of lines about reparenting blocks.

If you use Sync then best is to disconnect Sync temporarily during troubleshooting.

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

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 about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox