That is strange. I don't think it should appear when Firefox is started normally. Have you already tried exiting out of Firefox and starting it up again to see whether it goes away?

I think that icon would appear when Firefox was started by a testing tool that manipulates the browser to perform certain tasks. Or perhaps when certain developer tools are in use.

Or maybe when accessibility programs are manipulating the browser? But in that case, there should be a different icon also visible: What is the Accessibility Service Indicator? Do you see that one?

If you use custom style rules in a userChrome.css file to modify Firefox's user interface (that's not part of Firefox, it's something you add), that might be a factor.

Boot the computer in Windows Safe Mode with network support to see if that has effect.

• https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/
• https://www.7tutorials.com/4-ways-boot-safe-mode-windows-10