Excellent! This add-on is exactly what I was looking for without knowing it!
Very good job!

Just a small problem: at first, the feature does not work on some page, until I check / uncheck ALL options at least once (!).
Error as seen in the console :
TypeError: prefs.copyToClipboard is undefined (same with "prefs.alwaysHighlight")
[Firefox 62.0b15 (64 bits, on Ubuntu Linux 18.04), HighlightAll 2.1]