Nearly. There's a pull (merge) request for Linux support on the GitHub page. Once that's fixed, it should be possible to compile it for ARM (as long as all the dependencies are met).

Mobile Firefox has a very different UI to standard Firefox, so I guess a successfully compiled XPI would need some changes. It'd be easier to get it working for Iceweasel in Easy Debian.

What I'd do instead (if I had the time) is write a GUI program that filters Wireshark captures for the HTTP logins and extracts the cookies. You could then have a simple local HTML/JS file that's the GUI links to that sets the cookie in MicroB.