Hello Antoniou,

Please see this thread :

https://support.mozilla.org/en-US/questions/1234450

and this bug report :

https://bugzilla.mozilla.org/show_bug.cgi?id=1492459

hi, it appears that your avast/avg security software has wrongfully deleted important parts of the browser that are necessary for proper operations.

in order to recover, please try to start firefox into safemode by pressing the shift key while it's launching. if you get it to open up normally this way then please enter about:config into the address bar (confirm the warning hint in case it shows up), and search for the preference named intl.locale.requested - right-click and reset that preference and then try to start firefox normally again.

Diagnose Firefox issues using Troubleshoot Mode

Thanks guys

I have tried to do as philipp says but cannot find the preference named intl.locale.requested. The list skips from intl.keyboard.per_window_layout to intl.menuitems.alwaysappendaccesskeys.

hi raul202, in this case another thing should help you get back to a working browser: while in safemode, enter about:support into the address bar and hit the button to open the profile folder. then close all running firefox processes and delete the file named addonStartup.json.lz4 inside your profile folder, now firefox should be able to start up normally again.

Thank you Philipp, you are a star! All is working perfectly now :)