"eviscerate" was the first word I wanted to look up.

That was the message I got:
"The free version only supports English word lookups.

Wordwise defines any word in any language, and saves the sentence you're reading to Anki automatically."

PS: I installed the addon today via mozilla.org and I got this message so I guess I have the recent version.

When I install an addon and the very first interaction is a "Subscribe for money"-screen, that is not a good look.

Appreciate your quick reply, added a star.
I will try the addon and see if it does a good job for English vocabulary or if I run into "pay to use me", even for English. I will reevaluate in 2-3 weeks and may update my rating.