Currently running Firefox 54.0.1. Add-on failed to work when trying ANY of the functions with Netflix. Download and waste your time trying to make it do something, then waste MORE time removing it, and (hopefully)writing a scathing review about it's complete uselessness....
As I hovered over different movies on my Netflix page, the supposed RT and IMDB review scores would appear. However, when I clicked on them, they showed me that the score being shown was in fact for some completely different movie or TV series. It's probably not a coincidence that the show/movie being scored was somewhere else on the Netflix page I was viewing. So whatever I see in the hover box for a given Netflix movie can't be trusted at all, as it's almost always for a different movie. I removed the Add-On. What a shame - I really wanted this to work. It would be a great feature.
It's better than nothing, but its performance is erratic. About 1/3 to 1/2 the time it comes up empty (no ratings displayed at all) even though Rotten Tomatoes has ratings available under that exact same title.
The other issue is occasionally it will provide the wrong ratings. I'm only likely to detect this in a case where a known, highly-rated film has unexpectedly low ratings.
This is a really simple extension that adds a great function to Netflix. Excellent!
One word of warning, though - if you use NoScript, it'll work when you first install it (maybe because of the "No Restart"), and stop working immediately after. You'll need to whitelist "omdbapi.com" to get it working.