Revert to stable Firefox

[Chris Warburton]

Firefox has been crashing A LOT recently, possibly due to upgrading to
NixOS 17.09. Unfortunately we can't just use nixpkgs1603.firefox because
there's some sort of impure dependency on libGL.so, and we get a
'missing symbol' error when we try to run it.

Note that this also affects Conkeror :(

We should do the following:

 - Find a reliable way to reproduce the FF/conkeror crash.
 - Test this in a derivation, where we can pass in a different FF
   version and try the test.
 - Use 'shouldFail' or equivalent to ensure that super.firefox crashes
 - Test that the old Firefox doesn't crash (this will presumably take
   the libGL error into account too)
 - Find some reasonable way to make the old FF build against the new
   libGL.so
 - Provide the rebuilt old FF, using withDeps to ensure that it doesn't
   crash on the test case (including on startup, due to libGL) and that
   the latest FF does crash

Another reason to use older FF is for the extensions like DownThemAll.
We could also have a test check if FF supports them, since they don'
work in newer versions.