[CentOS] Firefox 46.0 fails on CentOS 6.7 due to GTK3 requirement

> I should have the CentOS-6 (and CentOS-5) version of Firefox 45 out in a
> couple of minutes .. currently building metadata and testing them on
> https://ci.centos.org/

Btw. this is the first ESR after the controversial removal of the "ask me every time" cookie policy. When the new browser is started for the first time, it will reset cookie policy to the default "keep until they expire".

Something to look out for