Rated 2 out of 5 stars
Provides some minor convenience in some situations. Needs some tweaking I think:
Can't add a word to highlight, so you can't highlight two different words unless they appear next to each other somewhere.
If you hit Ctrl-F you can highlight search a single word or phrase anyway. (But can't search multiple words separately.) If you could add new highlights to the list this would be much more useful, and provide functionality that isn't already in Firefox.
If you hold CTRL when you click "Highlight" on a word or phrase, existing highlights are not removed. By using this method, you can highlight multiple terms, regardless of where they are on the page.
I have no plans to add word lists to this particular extension; it doesn't really fit in with the concept. I may release another extension that does this, however.