Is that the full error message? Or does it say ""Firefox.app can't be opened because the identity of the developer cannot be confirmed"

If it says that, then after you try opening it, go to

1. Click on the Apple icon at the top left corner of your screen and open System Preferences.
2. Click on Security & Privacy, then select the General tab.
3. You will see a message stating that the Firefox.app was blocked. Click on Open Anyway.

If that's the full error message, "Firefox can't be opened" let me know.

Thanks. I tried that but there was no message when I opened the General tab in Security & Privacy.

I started to receive the same error message today. Interestingly I'm able to get Firefox to launch via the following methods:

1. Run "/Applications/Firefox.app/Contents/MacOS/firefox-bin" from a Terminal window
2. Run the Profile Manager via "/Applications/Firefox.app/Contents/MacOS/firefox-bin -P" from a Terminal window and launch Firefox from there
3. Go Applications in a Finder window, Control click on Firefox and select "Show Package Contents" then navigate to Contents -> MacOS and run "firefox-bin"

FWIW, when I uninstall / reinstall I am presented with the following popup (to which I click "Open").

One more data point. The Developer Edition of Firefox (72.0b11) launches without issue. The Beta version (also 72.0b11) exhibits the same behavior as the current default Firefox (71.0).

I have this same issue. I've used Firefox on my Mac for years; it's the preferred browser on all my computers, MacOS, Windows, and Linux. A few nights ago I was forced to remove Firefox from my MacBook because it posted a splash screen saying it had been compromised and that files were possibly being encrypted. Talk about shock, horror and dismay, and I wasn't even using Firefox. It was open, but I was working in the Inkscape 64-bit Beta release at the time.

After deleting Firefox from my Mac (10.15.3) and running a complete virus scan, all looked well, so I reinstalled Firefox from scratch. The installation completed normally, but when I attempt to open, I get the same message, "The application "Firefox.app" can't be opened." I tried starting in "Safe" mode, but get the same message. Any ideas are greatly appreciated.

Is it possible that the Gatekeeper settings are too strict ?

See also:

• https://support.mozilla.org/en-US/kb/firefox-cant-be-opened-after-you-install-it-on-mac
• Gatekeeper: https://support.apple.com/en-us/HT202491

Thanks for the suggestions Cor-el, but no joy. I even added Firefox to my firewall in the Security settings thinking that was the issue, but it didn't help.

Firefox isn't available through the the Apple App store, and I'm thinking I might be getting the 32bit version when I download from Mozilla to my MacBook. That's a definite killer running Catalina. However, I didn't see any explicit listings for Catalina, just the typical MacOS link.