You may be able to get the libfreshwrapper-pepperflash.so (not a link but file name) plugin working on Firefox as it allows you to use the Pepper version of Flash that Chrome comes with. I have used it by putting a copy in .mozilla/plugins/ folder so Firefox and SeaMonkey can use it with user. It worked okay but was buggy when trying to do some sites or say popout windows on twitch tv. At least it is a options since Adobe only supports the (plugin based) 11.2 esr branch version for Linux with security/stability fixes.