You should be on version 125.0.2 so something is wrong with your updates.

Does it still happen with Developer Edition?

You can try installing Firefox from the Microsoft Store which uses a different update system.

Do you only have one Firefox version installed and not possibly a 32-bit and a 64-bit version ?

• (64-bit Firefox) "C:\Program Files\Mozilla Firefox\"
• (32-bit Firefox) "C:\Program Files (x86)\Mozilla Firefox\"

Do you have NordVPN? You might need to disable NordVPN Web Protection.

I don't use Nord VPN I'm afraid, so this isn't it.

I have noticed an error when I first log in to Windows, about an XUL Runner being an old version. Google tells me this might have somethign to do with Firefox autoupdates, so I'm wondering if this could be part of the problem?

Firefox claimed it had updated itself the morning after I posted the original message here, but now I'm checked the version and it says I'm still on 124.0.2, so I suspect it's something to do with updates. I've also had an XUL Runner error when I log into Windows, I think this is something to do with updates?

I've checked whether I have two versions of Firefox installed (thanks cor-el), and found the following state of affairs. The following folders exist:

C:\Program Files\Mozilla Firefox C:\Program Files (x86)\Mozilla Maintenance Service

There is no C:\Program Files (x86)\Mozilla Firefox, nor is there a folder called C:\Program Files\Mozilla Maintenance Service. This strikes me as odd.

Thanks everyone for the suggestions so far. I feel they've helped me start to narrow down the issue. I closed Firefox but then needed to open it again a few minutes later, and it's updated itself again ('What's new' tab open and showing version 125.0.3 64-bit).

I'm wondering if the best way forward is to uninstall it and reinstall from scratch - any advice on how to preserve bookmarks, saved passwords, etc?