After you download and install Firefox on your Mac you may see the error message, "Firefox.app" can't be opened because the identity of the developer cannot be confirmed when you try to open it.
Apple has made a security change to Mac OS X 10.9.5 which may cause this error and block you from using Firefox. Here are some steps you can take if this happens:
Option 1 - Recommended
- Hold down the Control key while clicking on the Firefox icon in the Dock or on the Firefox application file in a Finder window.
- Click Open in the context menu.
- Click Open in the following dialog window.
See OS X Mavericks: Open an app from an unidentified developer for more information.
Option 2 - Open Firefox through the System Preferences
- Click on the Apple icon at the top left corner of your screen and open System Preferences….
- Click on Security & Privacy, then the General tab.
- You'll see a message stating that the Firefox.app was blocked. Click on Open Anyway: