hi, you're on beta or devedition with a language pack that became incomplete. try starting safe-mode by pressing the shift-key while firefox is launching, open about:addons and check if you have language packs installed. try to update them manually, or remove them, then restart normally and your firefox is hopefully recovered.

can not install firefox or open it

works fine on other computers in home

I just got this same error Daerius posted (Win 10 here) after updating to version 70. Holding the shift key before starting Firefox allows you to start Firefox in safe mode. I tried disabling all add-ons but that doesn't solve my problem. I went back to version 69.0.3 and that solves the problem here. When I update again to version 70, the exact same error returns.

Be sure to start Firefox while using the -allow-downgrade switch. I don't know how to use that in MacOS.

Edit: I missed the tip about the language packs. Disabling both English and Dutch worked. After re-enabling them both the error doesn't return.

I used CCleaner Pro to uninstall Firefox and then did a new install. It worked good but no additional tabs running, which I use 3 or 4 on 5 screens. I had to go to community support and find someone to help me. They showed me step by step on how to change programmings and code on Moz://a software.....wow I had to toggle the settings. They were opening right but needed a reboot.

I think I had a virus. I did two scans by Norton and AVG. nothing. I also did system restore..nothing Tried three times to install Firefox over old program..nothing Then had Moz removed by third party software Then reinstall ...works great