The application “Firefox.app” can’t be opened.

Greetings, LEScott.

Thank you for trying that and getting back to use, LEScott. It's appreciated. Since it still doesn't work since you reinstalled the macOS, I'd like you to contact Apple Support.

Apple - Support - Product Selection — After selecting this link, choose whichever type of Mac you're using, then type "application" in the Search topics field and select "Application does not respond or quits on launch" from the results. Then choose how you'd like to start the conversation.

Cheers!

Greetings, LEScott.

I see that you're unable to open the FireFox browser n your Mac on macOS Catalina. Good job in the troubleshooting you've tried, and I'm glad to help with this.

How to use safe mode on your Mac -- Since you couldn't open the browser in Safe mode, did you follow the steps under "If the issue continues in safe mode". See if reinstalling the operating system works.

Let me know how that goes.

Take care.

Firefox 75 works just fine in Catalina 10.15.4, as that is the current browser during this post. Have you introduced a new Firefox add-on, or is one of your installed add-ons 32-bit which would be an issue with Catalina? I am using a clean Firefox with no Add-ons of any kind.