Is FireFox provided within an appstack?
My guess is that Windows 10 in all it's wisdom decides to revert back to the default browser because it cannot see Firefox when logging in and sets it back to the default browser.
Mine is set to google but google is installed within the golden image
I've actually got it installed in the writable. And like you I have Google set as the default browser on the gold.
any particular reason you don't also have Firefox (ESR) included in the gold?
I only have a handful of users that use Firefox so adding it to the gold would only add further browser confusion to my user base at large. I'm also not sure that it would make a difference with the default browser problem since by default I have Chrome set on the gold.