I've updated everything I can update, safe mode makes it crash instantly on startup instead of after about a minute. Disabling hardware acceleration changed nothing. Tried 2 fresh installs and a new profiIe. Done 2 full scans for malware. I can't see anything in the event viewer at least nothing obvious that matches the times.

Do you run any "anti-exploit" programs, or does your security software have an "anti-exploit" feature? These programs attempt to inject code into Firefox processes and Firefox typically tries to resist, and that sometimes prevents Firefox from starting properly. If one of the popular programs has become incompatible, there should be numerous posts about it right around now (I haven't been reading due to the holidays, so I don't know).

More generally, I know this is a weird suggestion, but... will Firefox run if you rename its .exe file? Here's how to test:

In Windows File Explorer, open the installation folder, which usually is:

C:\Program Files\Mozilla Firefox

Right-click firefox.exe and rename it to firefox1.exe (giving Admin approval if required).

Then double-click firefox1.exe to run it.

Any difference?

This tests whether an external process is looking for and either killing or interfering with processes named firefox.exe. This can be a sign of a malware infection.

I thought malwarebytes was doing anti-exploit, but renaming the executable has done the trick. Obviously this isn't a long term solution, so what should be my next step.

Your Microsoft account may have parental controls enabled which limits what browsers you can use, you could try to get this limitation removed or create a new account.

Note that DRM protected videos will not work with a modified executable name.

How would the parental controls have been enabled? I certainly never turned them on. Can anyone recommend something other than malwarebytes, as repeated scans in that have found nothing. Thanks for all the help I've received so far.

Login to the Microsoft site to see how you account is configured. If it is a member of family group, "Family Safety" features like content filtering and browser blocking can be managed by other accounts.

Try creating a new local Windows user account.

No family group, and it's still closing on a local account.

Try rebooting in Windows Safe Mode with networking enabled.

Yeah, it works fine in safe mode, tried scanning in safe mode but nothing came up.

Do you play any games in the same system with AntiCheat installed?

Try uninstalling anticheat + rebooting the system.

One of such games with anticheat is `EA-FC24`.