EDIT: I realized my fix didn't correctly update and fix Amazon Prime Video after testing it out. I just tested it in both Firefox and Chrome and it is correctly working. Please use the new hotkeys to change video speed, left and right bracket keys below the backspace/delete key, and the backslash key to reset speed to 1.0x. LMK if there are any other sites that break for you as I haven't yet built out automated checks for every website where this extension may be used.

I downloaded the latest updates from my Github and packed them for firefox and submitted it to Mozilla for review. Hopefully the sites all start working for you after they approve and release version 4.1.0, which includes some cool features that allow it to speed through ads, instagram reels, boring corporate training videos, etc, as well as remapping the arrow keys to not collide with controls on other video sites.
From time to time sites like Netflix or Prime Video change the layouts of their HTML and classes or IDs, so the latest updates to the extension target elements by what they are (video elements) not just what they are named or what classes they are assigned, so hopefully it is more resilient to changes in the future. If not, submit another review as a bug report and I'll fix whatever is breaking.