I have definitely enjoyed this addon, and it works great (except for a few quirks on the nightly versions of Firefox, however I am perfectly alright with that, as it was my decision in the first place), I also put it on my family's PCs as a result.
I have noticed that it doesn't work on Yahoo Mail's latest beta (I have verified this on the stable version of Firefox) , and only because I have uBlock Origin installed, the advertisements themselves don't show.
While I don't use Yahoo as often, ever since I had moved over to ProtonMail, it would be nice if WebMail Ad Blocker would add support for the new beta interface on Yahoo Mail.