So I finally did get this to work on Android.
After many tries to edit Firefox to bypass the annoyingly rigid version checking it does for extensions, I got it to work with Opera for Android.
You need a specific and outdated version of Opera liked from their blog
here. (newer versions of Opera are a Chrome fork and the "Opera Classic" on Google Play apparently doesn't support extensions in that version).
Then you just install it via the Opera extensions page.
https://addons.opera.com/en/extensions/details/enhanced-steam/
It's a slighty older version of ES but eh, it works and my stuff is highlighted now on Android. At least if I got to the full desktop page. The
Steam mobile site doesn't highlight but that's okay. One thing at a time.
This works for me for now, though if the Firefox permissions get updated to allow Android I'll definitely be giving that a try again at a later point in time.