If firefox cannot play video it can be due to missing codecs. It is always good to start from determining a reason of problem and Firefox web console is right tool here (main menu / Web Developer / Web Console). I am using openSUSE and in my case h264 was missing. You can verify it using html5test too. I found nice related info what helped me to solve problem on Firefox_MP4/H.264_Video_Support.

I had used Packman repository - even when I am using tumbleweed - and installed every libavcodec, libavformat, libavdevice I found in actual repositories:

zypper in libavcodec56 libavcodec57 libavcodec58_91 libavformat56 libavformat57 libavformat58_45 libavdevice56 libavdevice57 libavdevice58_10

More info about installed packages can be found using -s (show details) and -i (only installed packages) switches:

zypper se -si libavformat

S  | Name             | Summary                        | Type
---+------------------+--------------------------------+--------
i+ | libavformat56    | FFmpeg's stream format library | package
i+ | libavformat57    | FFmpeg's stream format library | package
i  | libavformat58_45 | FFmpeg's stream format library | package

Btw i and i+ in first column means:

i+ installed by user request

i installed automatically (by the resolver, see section Automatically installed packages)