You can use one of these to start Firefox in Safe mode:
On Windows, hold down the Shift key while starting Firefox with a double-click on the Firefox desktop shortcut
On Mac, hold down the Options key while starting Firefox
Help > Restart with Add-ons Disabled
Another possible cause is security software (firewall,anti-virus) that blocks or restricts Firefox or the plugin-container process without informing you, possibly after detecting changes (update) to the Firefox program.
Remove all rules for Firefox and the plugin-container from the permissions list in the firewall and let your firewall ask again for permission to get full unrestricted access to internet for Firefox and the plugin-container process and the updater process.
Re: Firefox won't start up, when clicked, whereas every other browser works.
This reply was not helpful because firefox will not start. I have spent hours researching this problem without any progress, I have now resorted to my reliable I.E. which, fortunately is still working ok.