Description of problem:
It seems that https://bugzilla.mozilla.org/show_bug.cgi?id=1005640#c63 didn’t fix the problem on Fedora.

Version-Release number of selected component (if applicable):
firefox-57.0-2.fc27.x86_64, although it’s there for many versions now.

How reproducible:
Always.

Steps to Reproduce:
1. Launch Firefox with LANG=pl_PL.UTF-8.
2.
3.

Actual results:
en-US,en;q=0.5

Expected results:
pl,en-US;q=0.7,en;q=0.3

Additional info:
Doesn’t happen on upstream builds or Arch packages, as verified by a Mozilla dev: https://bugzilla.mozilla.org/show_bug.cgi?id=1005640#c67