Please explain the problem in detail. What happens? What is/are the exact error message(s) ?

Is that an add-on or a webpage?

Add-on; Type about:addons<enter> in the address bar to open your Add-ons Manager. Hot key; <Control> (Mac=<Command>) <Shift> A)

In the Add-ons Manager, on the left, select Extensions. Locate that add-on and remove it.

It seems I have resolved the problem. Thanks.

Hello,

I am glad to hear that your problem has been resolved. If you haven't already, please select the answer that solves the problem or explain your solution. This will help other users with similar problems find the solution.

Thank you for contacting Mozilla Support.