The same guy who made the extension for Chrome did not want to make it for Firefox. If you download the zip file of this extension, you can literally open the script and see the code for yourself. It's only 25 lines of code that get the video player in a YouTube page and sets the speed to x16 of the ad.

Stop lying about it having malware, ALL extensions are manually checked by Mozilla before being allowed to be published.

edit 1:
if you go to about:addons, click on gear icon and choose "install add-on from file" and choose any add-on in it's archive form, it says "this add-on cannot be installed because it has not been verified". So I had to publish the extension in order to use it fro myself, my initial intention was not to make it public, just to make it work for myself, which apparently requires me to publish it first and for it to get approved. You can try for yourself to install an add-on in archive form, get the same result, then try to publish it here and wait a few days for Mozilla to approve it.

edit 2:
I don't know JS, but I have knowledge about other programming languages, to me the code is super simple and works pretty straight-forward. In the extension are only a few files, the script and some icons, so I don't know what would you consider malware.