Firefox will also rewrite YouTube embeds: https://searchfox.org/mozilla-central/rev/c6d17806faef8708b6511e9849132463b7ea6c37/dom/base/nsObjectLoadingContent.cpp

Firefox will also rewrite YouTube embeds: https://searchfox.org/mozilla-central/rev/c6d17806faef8708b6511e9849132463b7ea6c37/dom/base/nsObjectLoadingContent.cpp#846 https://searchfox.org/mozilla-central/rev/c6d17806faef8708b6.... Firefox also ships a "system" add-on (https://github.com/mozilla-extensions/webcompat-addon https://github.com/mozilla-extensions/webcompat-addon) that has interventions and user-agent overrides for many websites. See for yourself in Firefox on about:compat.