Switch stable Firefox back to GitHub releases because APKs differ from Firefox CI?

In researching #28 (closed), I noticed that the stable Firefox APKs from Firefox CI (which we're currently using) now differ from the APKs from GitHub releases: for example, the classes2.dex file is actually different. I don't know how long this has been the case or how significant the difference is to users. But since there's a strong argument to be made that the GitHub releases are a more official source than Firefox CI and it looks like Mozilla has a new process that publishes APKs to the latest GitHub release as soon as it is created (so we probably won't run into the missing-APK problem again), we should consider switching to GitHub releases, at least for stable Firefox. For Firefox Beta, that would be harder to do because we can't just query GitHub for the latest release; we'd need to add code to download.py to look through all the releases and find the latest beta release. Keeping consistency with Firefox Beta and FFUpdater (which both use Firefox CI) would be a conceivable argument not to switch stable Firefox to GitHub releases.

@genodeftest What do you think?