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!

I always used the vocabulary highlighter, so I am really happy to discover this add-on. But I have a problem, the restriction of three characters for the word we could highlight. For example I want to highlight this word: 六木
and add the description box: Mutsuki. But it is not possible with this add-on, could you do something about this?

I would like to use the add on, to mark different recurrent sets of words to organize my workflow. Seems to work pretty well. The add-on would be of even more use, if there was a way to mark certein setzt of information belonging together in a certain colour and other sets in different colours. For Example I have ten different word I would like to mark red and tenn I would like to mark green, because they lead to different decisions and actions within my workflow. Up to now I used the "multiple highlighter" tool which does not work any more with Firefox Quantum and I cannot find an alternative for that. Your tool would work great with the option of different colours.

Updating my review.....

This extension does a great job in highlighting multiple keywords in ALL pages.
So far, this is the only multi-page keyword highlighter I've seen for firefox 57+.

It has a great user interface, which is very intuitive.

You can use it as a multi-page keyword highlighter, and you can use it to refresh your memory for definitions of keywords, acronyms, or phrases.

Programmers, Engineers, and Military personal all have to deal with too many acronyms, and it's tough memorizing them all.

By using this add-on to create a list of keywords with their definitions, the user can hover the mouse over a predefined word and get a popup text with the definition.

It's great for programmers learning a new programming language, or those have have to dabble in multiple programming languages.

It's a good tool for anyone learning something new in general.

For example, I'm currently learning 3D Printing and CAD, which have many new terms. I'll read the definition of a new term, and then I don't see again until 2-4 weeks later. By then, I may not remember the exact meaning.
With this tool, every time I see a new term, I can add it to the list with the definition, and the next time the word comes up, I can use the tooltip to quickly get the definition.
Once I feel familiar with the term, I can easily deleted out of the list.

This tool is great as-is (5-stars), but I have some recommendations:

1. Add shortcut key to enable and disable highlighting.

2. Add functionality to include multiple words in one item. Use comma separation to distinguish each word.

3. Add feature to group multiple keywords and definitions, and include ability to enable and disable each group. Let user be able to customize highlight color for each group.

4. Add ability to have temporary keyword(s), which apply to all tabs. This can be done with existing GUI, by just letting the user enter the keywords in existing field, but not clicking the "Add" button. Maybe add an additional button "Apply to Newly Loaded Pages". With this change, I would also rename the "Add" button to "Save".

5. Let user customize color for each word.

6. Instead of having "Strict Match" option, break it down to two options (case match and whole word). Also consider having start of word and end of word option. Recommend using a combo box for start-word, end-word, and whole-word. The just have a switch for case match.

7. Have option to automatically apply different highlight colors to each keyword.

8. Add option to remove confirmation when deleting keywords.