Nice and usefull add-on!
Lot of thanks to the author!
It could be a much more popular if has more features:

1. Enable/disable add-on with 2 clicks on toolbar icon.

2. Ability to add/remove/export/subscribe many vocabularies and enable/disable them.

3. Subscribe feature would be great! It is just ability to import vocabulary from URL of text file.

4. Assign custon color of highlited word and background/text of description - for each vocabulary separately.

Works great in general, a good replacement for the now-dead "Vocabulary Highlighter" add-on which I was using before. I had one problem in that I tried to import 2,500 words and they all appear in the word list but on every page only 5-10 are highlighted. I don't know if this is my fault or a fault in the add-on. I have some suggestions:

1. You should be able to import/export your wordlist via a spreadsheet file or something similar. I'm gathering words to make my own dictionary, so being able to easily export and import all my words is very important. Right now you can highlight and copy-paste your wordlist from the add-on into a spreadsheet (thanks so much!!), but you can't import from a spreadsheet.

2. It would be nice if you could change the highlight color. I use another add-on (for people with bad eyesight) that changes all pages to black with white text, but reading white text with yellow highlight is really hard. Other people might want pink highlight for nouns, green highlight for verbs or suffixes, things like that. To other readers: if that option never happens, you can change the browser's highlight colour yourself using Stylish and some CSS.

3. When you first add a word via the right-click (context) menu, you should be able to add the definition at the same time. Right now it adds the word but the definition is blank so you have to immediately click on the add-on icon, click edit for the most recent word and then add the definition. Also right now it's hard to edit words you've already added that have wrong definitions, it would be better if you could right-click on the word on the page and edit it, or type it into a searchbar inside the add-on and it will find the one to edit for you.

4. It would be nice if you could claim one entry was the same as another entry so you didn't have to type the definition in a second time. Ex. "aqua" and "water" would both have the same definition as "water". This is for adding in things like abbreviations, common spelling mistakes, dialectal or historical versions of a word, etc. For example, I'm learning a language with no set spelling system, so the same word is spelled 10 different ways and I have to find and copy-paste the definition every time I find a new spelling!