Does Firefox appear in the App Switcher? If you hold down the Command key and then press and release the Tab key, MacOS should display a bar of running programs that you can switch to by pressing Tab additional times. Is Firefox listed?

If Firefox is listed, if you keep the Command key pressed and press Tab enough times to select it, and then release both the Tab and Command keys, in theory, Firefox should be made the active application.

Does anything appear to happen on the screen?

If not, is there any possibility that Firefox is open on a different display? Obviously only applicable if you use (or have used) multiple displays.

I finally just reinstalled Firefox, and things seem to have stabilized. Thanks for the tip on the running programs!

I have had the same problem appear twice, and found the linked solution to work in both cases: https://osxdaily.com/2010/05/16/fix-for-close-firefox-a-copy-of-firefox-is-already-open-error/

Doesn't require a reinstall, but basic knowledge of Terminal and Mac OS file structure.

I have this same problem -- Firefox is not running in the app switcher (or on another display), and I tried deleting and re-installing it. Tried going into the Terminal to run Emile's solution but i don't really know enough to make it work. Any other suggestions?