The latest version of firefox, 46.0 requires GTK3 and so it fails on CentOS 6.7.
I know there is the ESR release supplied by upstream, but that is based on version 38. We have regularly installed the Mozilla "Linux" version of Firefox in a central location for our users, and this has worked just fine until 46.0. (I note from https://www.mozilla.org/en-US/firefox/organizations/faq/ that even that ESR release is due to stop being supported soon).
I filed a bugzilla report at https://bugzilla.mozilla.org/show_bug.cgi?id=1267815. If this matters to you, please consider tagging onto that bug.
Yes, yes we have to upgrade to CentOS 7. But in our shop, the amount of work required to get this done is huge, and won't happen soon.
The whole idea of using RHEL/CentOS is to have a stable, supported version for a long period of time. How can we do this if even a web browser won't work?