The "fancy update policy" being 'latest FF stable uses latest rust stable', which seems like a pretty reasonable policy to me.
If you are maintaining a large number of machines in an enterprise environment, you're not keen on updating half of the installed system just because you update your browser, simply because the necessary testing and fixing of regressions costs a lot of time and money for no real gain for both users and administrators.
Have you ever heard of any program requiring a 6-weeks-old-or-newer version of GCC or Clang, and failing to build on older ones?
"Stable" is just a name -- Rust stable is anything but.