Recent versions of Iceweasel should have the OpenH264 decoder plugin available in the plugin menu. Try purging it and re-installing it. Don't forget to get rid of your local configs too. What versions of Iceweasel are you running? I run stretch, and the decoder appeared in my plugins menu after I installed. I believe both Stretch and Jessie have the same version of Iceweasel (38, notwithstanding trivial version numbers).