For example, continue this person's work...

http://www.webupd8.org/2014/05/fresh-player-plugin-pepper-flash.html

I doubt if Mozilla will ever use the Pepper plugin for Firefox. Mozilla is working on a project named Shumway to eventually replace Flash, which works with all operating systems unlike Fresh player. http://mozilla.github.io/shumway/

The Web Extensions API for Firefox is just a proposal at this time. I don't think any work on it has started in earnest and it could be 12 to 18 months away from even alpha testing. Similar to how Electrolysis has been a couple of years in the works already and still isn't near the alpha 2 stage - Aurora / Developer.

From https://bugzilla.mozilla.org/show_bug.cgi?id=729481#c83 - Support the "Pepper" Plugin api

Boris Zbarsky [:bz] 2014-11-09 This has been discussed to death on the technical discussion mailing list. Let me summarize the most salient points of that discussion, for those not willing to go read it themselves: 1) There is no actual clear definition of the Pepper API. It's basically defined by Google's implementation. There's API documentation, but that doesn't cover edge cases, and it's quite likely that actual plugins written against the API rely on the edge cases. 2) Google's implementation is very much tied to Blink and is not designed to work with any other rendering engine. 3) The Flash plugin in particular uses a ton of non-public Pepper APIs that are not even mentioned in the Pepper API documentation. Anyone commenting on this bug who hasn't bothered to go read those discussions and understand these points and their implications is wasting not only their own time but also that of everyone cced on this bug. Given the number of people who seem to be intent on doing that, at least 7 people (counting myself as of this comment) have removed themselves from the cc list of this bug in the last few days, because we don't particularly like having our time wasted. The end result here is likely a "why isn't Mozilla adding support for Pepper?" echo chamber, because I doubt new people showing up here will bother to read either this comment or the mailing list discussion....

Linking to bug since new comments are restricted on this bug so no worry of additional spam with "Why?" comments by people who do not follow the Bugzilla Etiquette and do not read before commenting.

The closest you will get to using the Pepper Flash player in Firefox and SeaMonkey browsers is by using the Fresh Player Plugin wrapper (on Linux only) to use the Pepper Flash that is a part of Chrome web browser.

I have tried the wrapper, not perfect but it works as a possible option since Linux users only get security updates for the old Flash Player 11.2 ESR branch from Adobe. The catch is you have to install Chrome in order to use this Pepper Flash plugin.

Many websites are using HTML5 as a option or switched to it. Sites like Twitch.tv have made change to HTML5 for the controls and chat and plan to use HTML5 for the video part. There is a new player in testing but it currently has no chat for the link I am aware of and still has Flash embedded for video.