hi, thanks for reporting this. 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

Does Avast have a log of what it does, or a quarantine that lets you restore removed files? That would be helpful in this kind of case...

I've put in about:config and it tells me 'Here be dragons only to do this if you know what you're doing! Do I continue??

yes, please continue - don't make arbitrary changes on that about:config page though but only what we were referencing here in the thread...

There isn't a intl.locale.requested. It goes from intl.keyboard per..... to intl.menuitems.always.........

ok, then another thing to try - 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.

sorry don't understand 'close all running firefox processes' so can't find addonStartup.json.Iz4

jscher2000 Any ideas how I find it in free Avast?

Ignore it's now sorted with the help of Philipp.

I don't use Avast.

Why can't you download Firefox? Do full installers work any better:

https://www.mozilla.org/firefox/all/

Philipp - Thanks I did eventually work it out with the help of Google to explain how I found running processes and I've now got Firefox back. The home page doesn't have all the apps etc showing in the boxes but I can get private viewing which is great for me!