Start Firefox in Safe Mode {web link}

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

The problem didn't occur in Troubleshoot Mode. After disabling all extensions, and re-enabling some, I narrowed the problem down to either or both Firefox Color and Notes by Firefox. Since Notes is no longer supported, it can remain disabled. I do miss the custom look I added in the Test experiment. Is there a similar customization option now?

I suggest you post a new question explaining what you want.