This is available only on Android though. On iOS, Firefox Focus (or any Firefox or other browser) is tied to Apple's restrictions. So there's no scope for browser extensions as we generally think of it.
And Mozilla is about to roll out Firefox Preview soo with all add-ons but uBlock Origin disabled until they can fix up and test the add-on ecosystem on the new browser. Unfortunately there's more add-ons for privacy/security/ethics than uBlock (Decentraleyes, tracking token strippers, et. al) which will include add-ons that redirect to the original source from AMP.