Make sure that you do not run Firefox in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History).

• https://support.mozilla.org/kb/Private+Browsing

• Tools > Options > Privacy > Firefox will: "Use custom settings for history"
• Deselect: [ ] "Always use Private Browsing mode"

Use one of these to close Firefox if you are currently doing that by clicking the close X on the Firefox title bar.

• 3-bar Menu button > Exit (Power button)
• Windows: File > Exit
• Mac: Firefox > Quit Firefox
• Linux: File > Quit

You can rename (or delete) places.sqlite files including places.sqlite-shm and places.sqlite-wal in the Firefox profile folder to remove all history. Firefox will create a new places.sqlite file and recover the bookmarks from a recent JSON backup in the bookmarkbackups folder.

You can use this button to go to the current Firefox profile folder:

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

Thanks for that.

cor-el said

Make sure that you do not run Firefox in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History).

I think I wasn't clear enough - permanent Private Browsing mode isn't enabled, but in the Privacy options, "Clear History when Firefox closes" is ticked and all of the options within that are ticked.

cor-el said

You can rename (or delete) places.sqlite files including places.sqlite-shm and places.sqlite-wal in the Firefox profile folder to remove all history.

The problem isn't that I can't delete history at all (manual clearing with Ctrl+Shift+Del works). It's that Firefox is set to delete history automatically and it doesn't.

Ironically, just shut down from the session typing this reply and restarted Firefox. History seems to have deleted on exit.

It didn't do that when I tested before typing the reply!

Looks like problem solved, although no idea how/why. Weird.