The /etc/profile.d/gmpopenh264.sh script[1] shipped with mozilla-openh264 subpackage sets MOZ_GMP_PATH without regard to existing MOZ_GMP_PATH value.
This interferes with other system-wide scripts in /etc/profile.d trying to add something to MOZ_GMP_PATH, e.g. to configure a system-wide installation of widevine-cdm.

[1] https://src.fedoraproject.org/rpms/openh264/blob/rawhide/f/openh264.spec#_130

The relevant line in /etc/profile.d/gmpopenh264.sh should read:

MOZ_GMP_PATH="${MOZ_GMP_PATH}${MOZ_GMP_PATH:+:}/usr/lib64/mozilla/plugins/gmp-gmpopenh264/system-installed"

instead of:

MOZ_GMP_PATH="/usr/lib64/mozilla/plugins/gmp-gmpopenh264/system-installed"

Reproducible: Always

Steps to Reproduce:
1. echo 'export MOZ_GMP_PATH="${MOZ_GMP_PATH}${MOZ_GMP_PATH:+:}/usr/lib64/mozilla/plugins/gmp-widevinecdm/system-installed"' > /etc/profile.d/cdm.sh
2. Install mozilla-openh264
3. Re-login
4. echo $MOZ_GMP_PATH
Actual Results:  
/usr/lib64/mozilla/plugins/gmp-gmpopenh264/system-installed

Expected Results:  
/usr/lib64/mozilla/plugins/gmp-widevinecdm/system-installed:/usr/lib64/mozilla/plugins/gmp-gmpopenh264/system-installed


This was masked by bug 2221317 in firefox launcher script, which is now fixed.

I can work around this issue by renaming my script so that it gets executed after /etc/profile.d/gmpopenh264.sh, but I really shouldn't have to.