Why are you using Firefox 51? The latest version is 66.0.5. Please install that version, then let us know if you still have issues

As a footnote to the first reply, see: Update Firefox to the latest release.

Note: if you upgraded to a newer version and THEN downgraded, some files in your profile may no longer be readable by older versions. However, moving to the current release might work around that. If that doesn't happen automatically, then we might be able to suggest things check for among your Firefox data files.

" we might be able to suggest things check for among your Firefox data files."

Waiting for suggestions! Thanks!

Hi JJJoseph, sorry for the delay, I was out of town for a conference.

What version of Firefox do you have now?

Have you already done a normal shut down and restart of your Mac since this problem started?

As you may know, Firefox stores your data in a profile folder. It is possible to have multiple profile folders, some accessible through the Profile Manager page, and some invisible to Firefox. Most likely, your old bookmarks are in one of those profiles, but it can take some work to track down and copy them over to your new/preferred profile.

Could you start with the backups in your current profile. This article has the steps: Restore bookmarks from backup or move them to another computer. Anything there useful?

To check other profiles that Firefox is aware of:

The Profile Manager page

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it. You may see one or more profiles on this page, each of which has two associated folders (the "Root" folders have the good stuff in them).

The currently active profile has the line "This is the profile in use and it cannot be deleted."

Do you see others? If so, you can check each one to see what bookmarks it has:

Click the Launch profile in new browser button for one profile. Firefox should open a new window that may look different from your current windows due to different toolbar layout or theme.

If you pop up the Library window (Command+Shift+b) in the other profile, does it have the missing bookmarks when you click the various categories on the left side?

If no: You can close the Library window and the regular window for this profile, then return to your about:profiles page and try any others.

If yes: Do you want to switch to using this profile as your default, or just extract its bookmarks?

To switch your default profile: return to the about:profiles page you left open and use the Set as default profile button for this profile. Then you can close all the windows. The next time you start Firefox, it should start up in the profile you designated as default. If you accidentally got the wrong one, you can use the about:profiles page to change again.

To extract bookmarks: Use the Export feature to export these bookmarks to a file on your Desktop for easy access. See: Export Firefox bookmarks to an HTML file to back up or transfer bookmarks. Then close this Library window and the regular window for this profile. In a Library window in your normal profile, use the Import feature. See: Import Bookmarks from an HTML file.

Any progress so far?