Are you using sync?

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.

Are you using any bookmark add-ons or programs?

Start Firefox in Safe Mode {web link}

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

[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.

That fixed it for now. Thank you.

What fixed it? 'For now' does not mean fixed permanently.

Undoing Sync with my two other computers and restarting in the safe mode that killed all the extensions that I had added. For now meant that nothing has happened in the last half hour, and so far it has stayed the same ever since I followed your suggestions. I did the same process on all three computers.

Leaving sync off, start Firefox normally. If there is no problem, give it a few days. Is the problem gone?

Type about:support in the address bar and press Enter. Under the main banner, press the button; Copy Text To Clipboard.. Now in the Reply Box on the forum page, do a right-click in the box and select Paste.

This will show us your system details. No Personal Information Is Collected.