I can't answer that, but I notice that the system requirements for Linux usually are stated in terms of specific versions of libraries and packages.

So for Firefox 41:

• Firefox will not run at all without the following libraries or packages:

       GTK+ 2.18 or higher
       GLib 2.22 or higher
       Pango 1.14 or higher
       X.Org 1.0 or higher (1.7 or higher is recommended)
       libstdc++ 4.3 or higher

• For optimal functionality, we recommend the following libraries or packages:

       NetworkManager 0.7 or higher
       DBus 1.0 or higher
       HAL 0.5.8 or higher
       GNOME 2.16 or higher

For Firefox beta (future Firefox 42) there is a significant change for the first item:

• Firefox will not run at all without the following libraries or packages:

       GTK+ 3.4 or higher
       GLib 2.22 or higher
       Pango 1.14 or higher
       X.Org 1.0 or higher (1.7 or higher is recommended)
       libstdc++ 4.3 or higher

• For optimal functionality, we recommend the following libraries or packages:

       NetworkManager 0.7 or higher
       DBus 1.0 or higher
       HAL 0.5.8 or higher
       GNOME 2.16 or higher

Developer Edition lists the same requirements as Beta.

There probably is a similar list for Nightly, but the download pages are organized differently and I didn't see it.

Yeah the GTK 3.4+ (three.four) is a pretty big change for Linux Firefox in last while and will likely get a number of posts in relation for Firefox 42.0 Release if not also Beta builds of 42.0 at times. Both on trying to run Firefox and for those who normally use GTK2 Themes instead of GTK3.

I use XFCE so I have mainly used GTK2 Themes and this is bit of a headache if I want to Theme newer Firefox and SeaMonkey builds on Gecko 42 and up from a certain date on. I have yet to find GTK3 Themes I like to switch to permanently over GTK2.

According to distrowatch the Red Hat Enterprise Linux 6.6 has GTK 2.18.9 at install.

I guess that may make Firefox ESR the best choice for GTK2 users when Firefox 42 is released? At least it's a temporary reprieve.

To be able to run Firefox with security updates yes the 38 ESR with updates would be a good option but for those who do have GTK 3.4+ but want GTK2 Themes when that is more cosmetic well.