This is likely just a temporary experience where the current stable release is technically labelled as "87.0.0-rc1". I agree that it's a bit confusing, but I think it is most likely temporary.
The latest official release version of Firefox for Android can be obtain here:
Alternatively, you can enter your email at the following page, and Mozilla will email you a link to the current official release version:
All the source code for Firefox mobile is available here (unlike many other browsers, there is no non-public source code used to create Firefox):
BTW, all releases of the desktop version of Firefox are available via FTP, but I'm not aware of anything identical for the Android version. For completeness, that FTP site for the desktop releases can be accessed via this link: