I agree with the sentiment (as a Firefox user who does not like the Chrome monopoly), but keep in mind this looks like a project purely for fun. The author owes us nothing and is nice enough to share their experiments. They may not have even noticed it only works on Chrome. We can certainly find a nice way to suggest them to specify that this is Chrome-only, maybe by sending a PR on the README file.

There's no need to be negative. You can even convey your message nicely.

Usually, people are receptive to feedback if the feedback is respectful and gentle.