www/firefox: update to 49.0 (rc3)

Mozilla shortened 51.0 Aurora and 50.0 Beta cycles[1] instead of
releasing 49.0.1 shortly after 49.0 which led to ESR security fixes
delayed as well for no good reason. The latter didn't affect us
because for a short time r421490 and r421531 traded stability for
security at the cost of only slightly confusing www/firefox users,
mainly due to this commit.

[1] https://www.mail-archive.com/dev-platform@lists.mozilla.org/msg19725.html

Changes: https://hg.mozilla.org/releases/mozilla-release/pushloghtml?startdate=2016-09-08&enddate=2016-09-13
MFH: 2016Q3