I can confirm this bug. I tried the same fixed like you did, as well as some other workarounds found on the net, but unfortunately none of them worked. I did also try to launch IceWeasel as superuser, but this didn't do anything as well.
On another note: why isn't OpenH264 included in the main repos? AFAIK it is licensed under BSD, therefore free software (or am I missing something here?) https://github.com/cisco/openh264 System information: Debian stretch/sid (testing) iceweasel 38.2.0esr-1-strech
