Since Adobe has stopped developing Flash for Firefox and Mozilla has no plans to support the Pepper plug-in API, Rinat Ibragimov is developing a wrapper to use Google Chrome's Pepper-based Flash plug-in with Mozilla Firefox:

GitHub: i-rinat/freshplayerplugin

A live ebuild is now available in the "betagarden" overlay. Users of non-stable Chrome can edit the plug-in path in /etc/freshwrapper.conf. Interestingly, freshplayerplugin makes use of my library uriparser for URI handling. Very nice!