Shared components used by Firefox and other Mozilla software, including handling of Web content; Gecko, HTML, CSS, layout, DOM, scripts, images, networking, etc. Issues with web page layout probably go here, while Firefox user interface issues belong in the Firefox product. (More info)
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1329543 - Remove Adobe Primetime supporting code
QA/Youtube Embedded Rewrite - MozillaWiki
1255918 - Disable YouTube Flash-to-HTML5 embed rewriter in Beta 46 (plugins.rewrite_youtube_embeds = false)
1209755 - [EME] Enable Adobe EME in official 64bit builds in Nightly, Aurora, and Beta
769117 - Rewrite YouTube object embed with iframe so we can play HTML video instead of Flash
Media/openh264 - MozillaWiki
957928 - Gecko Media Plugins (GMP) support