As with all the "releases", this is just marking a certain point in the state of the repo. To make use of this, you can download a .zip or .tar.gz archive of the repo using the asset links below. (And then start with the toplevel readme.)
Since I've switched to using the Firefox browser personally, I wanted to make a Firefox version of this extension. It was a bit of a journey but here we are.
So the main thing in this new version is the addition of the Firefox extension (the Chrome extension is still available). The instructions have been updated accordingly.
I've also done a minor tweak to the Chrome extension, removing an unneeded "scripting" permission in its manifest.