Not on ARM. There you only have a choice between Microsoft store apps or emulated x86 applications. You're not going to use Firefox on Windows on ARM unless you download the x86 version which will perform very poorly.

No, you can run native ARM apps sourced from outside the windows store.

Not true here is the download to the exe for Windows Firefox ARM build. Took me 10 seconds to Google it....

https://download-installer.cdn.mozilla.net/pub/firefox/relea...