Is there any equivalent or close to equivalent feature on other browsers?
Still pretty dope if it's firefox-exclusive, just curious to know if firefox-based browsers have it as well
Google has an app called Reading Mode that you can trigger by holding both volume buttons. Works anywhere and has dictation, but the output isn't as good as the Firefox one in my experience.
In addition to the other comment, if you hit a page that force disables it, sometimes you can see it if you stop the age mid-load, then go into Reader Mode, and refresh.