We're sorry to hear that Firefox is crashing when you open it. Please do the following to give us crash IDs that will help us understand more about the crash.

1. Click the Finder icon in the dock. On the menu bar, click the Go menu, hold down the option or alt key and select Library. A window will open containing your Library folder. Then open the "Application Support" folder, the "Firefox" folder, the "Crash Reports" folder and finally the "submitted" folder.
2. Open the 5 most recent files with a text editor and copy the IDs.
3. Paste each ID with bp- into the reply window on the forums.

Thanks in advance!

You can find more information and troubleshooting steps in the Troubleshoot Firefox crashes (closing or quitting unexpectedly) article.

So found the problem, was nothing to do with extenstions, plug ins or anything else.

Seems in 63.0.x builds Firefox needs to be in the Application folder under macOS X. I had FireFox in Application/Internet Apps ƒ and this broke FireFox from launching.

Fixed by moving the app just into the Application folder, not ideal but I can live with it, if you could provide this issue to the dev team please, hopefully they will fix this and allow the app to run successfully in the users chosen location.

Thnx, Steve