As Adobe no longer supports Flash on Linux (see "NOTICE" below), is there a way to use Chrome's Pepper Flash plugin with Firefox?
NOTICE! Flash no longer is supported for Linux EXCEPT as part of Chrome (with the exception of the extremely out of date Firefox 11.2 version, even though it gets security updates, it is still ancient and not updated that frequently). This is detailed in the Roadmap for Flash Runtimes from Adobe. The relevant section is Platform support, sub section Personal Computers, section Linux, quoted here (January 14, 2015 was when this was pulled, and the most important lines in this have been bolded for important reasons because people won't want to read the whole thing here):
Adobe has been working closely with Google to develop a single, modern API for hosting plug-ins within the browser. The PPAPI, code-named "Pepper", aims to provide a layer between the plug-in and browser that abstracts away differences between browser and operating system implementations. You can find more information on the Pepper API at http://code.google.com/p/ppapi/.
Because of this work, Adobe has been able to partner with Google in providing a "Pepper" implementation of Flash Player for all x86/64 platforms supported by the Google Chrome browser. Google now distributes this new Pepper-based Flash Player as part of Chrome on all platforms, including Linux.
For Flash Player releases after 11.2, the Flash Player browser plug-in for Linux will only be available via the "Pepper" API as part of the Google Chrome browser distribution and will no longer be available as a direct download from Adobe. Adobe will continue to provide security updates to non-Pepper distributions of Flash Player 11.2 on Linux for five years from its release.
Flash Player will continue to support browsers using non-"Pepper" plug-in APIs on platforms other than Linux.
Adobe will not be providing a Pepper-based debug player implementation of the Flash Player browser plug-in on Linux.
As of Adobe AIR 3, Adobe has discontinued support for Adobe AIR for Linux operating systems.