If doing development on any of the Mozilla applications, it's possible a user will set the MOZCONFIG environmental variable to use their own developer settings.
Even if no special settings are configured, the mere existence of this env var, if picked up during a compile of an ebuild such as Firefox or Thunderbird, can affect the result. In my case it simply caused Firefox to use the "Nightly" branding, but this is still undesirable.
Filtering MOZCONFIG in moz_pkgsetup() should eliminate any chance of this happening.
 Example of how MOZCONFIG is used: https://developer.mozilla.org/En/Developer_Guide/Source_Code/Mercurial
Created attachment 312909 [details]
Patch to filter MOZCONFIG
To speed things along, here's a patch that I tested in an overlay. It successfully ensures that firefox builds in a "clean" environment regardless of user env settings.
Thanks for reporting. I have pushed to overlay, and tree.
eval is not needed here.