Recently, I have been unable to delete Recent History either from the tool bar at the top of the screen or from Preferences. This is an older iMac for which Firefox is up-to-date. What is odd is that I am able to delete History when I use Safari as the active browser. I prefer using Firefox however, so what is wrong and how can I fix it?
If you have sync, and there is a problem anywhere, Shut Down Sync Immediately On All Devices to prevent the problem from spreading. Once the problem is fixed, perform the same repair on all computers/profiles before using sync again.
[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.