Look on your desktop. Do you see a folder called; Old Firefox? Look inside. Look for the folder with the latest creation date.

https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

This sentence caught my eye:

It is stopping me from bookmarking anything now.

Most likely the Places database that stores bookmarks and history is either locked or corrupted. If you haven't already, try doing a normal Firefox exit, and normal shutdown and restart of Windows to release file locks and resolve any transient permission issues. Then try Firefox again and see whether you can save new bookmarks or restore a bookmark backup (Restore bookmarks from backup or move them to another computer).

If not:

Let us know what you get when you run the "Verify Integrity" function. That's here:

Open Firefox's Troubleshooting Information page using any of these methods:

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

Near the bottom, there is a section titled Places Database -- that's the file which stores history and bookmarks. Click the button labeled Verify Integrity. There may be a 10-15 second delay before results appear.

If all is well, the report that appears below the button should start with:

> Task: checkIntegrity
+ The places.sqlite database is sane
+ The favicons.sqlite database is sane

Does yours say that, or something else? You can select and copy the report, and then paste it into a reply for review and comment.

The hard restart fixed everything after I spent a lot of time trying to rebuild it. Never again will I read these Firefox alerts. But I have a question for future reference: I found the Old Mozilla File on my desktop with the bookmark file, but I didn't know what to do with it should the restart not have fixed the problem. Do I copy and paste just the bookmarks and where do I paste the file? In my Mozilla program file? But what file in it? Thanks, for your help.

One or more of the articles Fred referenced shows the most useful files in your old profile folder. They would replace the same-named files in your live profile folder. For example, places.sqlite and favicons.sqlite for bookmarks and history (data and site icons, respectively).

I understand that, but how do I replace them? A simple drop and drag to the identical file in my Program files?

I prefer right-click > copy / right-click > paste. If something goes wrong, you'll want a second chance.

Also, not program files, profile folder: Profiles - Where Firefox stores your bookmarks, passwords and other user data.