By the way, Firefox is my system default browser. The team has put in a ton of work to make it fast and stable.
A table comparing the range of playback rates that you can set on HTML
|OS||Browser||Minimum||Maximum||Source Code Link|
|macOS v10.14.4||Chrome v74||0.0625||16.0||Chromium source code|
|macOS v10.14.4||Firefox v66||0.0625||16.0||Firefox source code|
|macOS v10.14.4||Safari v12.1||0.0625||2.0||I looked, but could not find where it’s defined in the WebKit source code. Maybe it’s in the closed Safari code?|
Modern hardware can play video smoothly at even very high rates. I guess Apple doesn’t think there are use cases for playing back video beyond double speed. Furthermore, their own documentation about the ability to specify playbackRate on iOS is inaccurate. As with desktop Safari, you can set it to the same maximum of 2.0, at least as of iOS 12.