Does this still happen in Safe mode?

https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Hmm, I'll be interested to hear the results of your Safe Mode test.

If you have problems adding or editing bookmarks/history, could 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.

In case errors are reported with "Verify Integrity", close and restart Firefox and retry.

If "Verify Integrity" cannot repair places.sqlite then rename/remove all places.sqlite and favicons.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

See also:

• 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

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

This might, or not, be helpful to a tech-savvy engineer. Unfortunately, I am just another dumb surfer and the whole post is therefore meaningless to me. Thank you for trying to help.

Start Firefox in Safe Mode {web link}

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

If it is,

[v57+] Places Maintenance is built into Firefox.

Type about:support<enter> in the address bar.

You will find Places Database near the bottom. Press the Verify Integrity button.

When done, copy and post the results here.

The advice to enter 'Firefox Safe Mode' and refresh is the best option. Fireox is back operating normally. Thanks everyone for your help.

Glad to help. Safe Surfing.