This does what it advertises: It will let you open a JS file in Firefox and display it with nice syntax highlighting. However, when I downloaded this, I thought it would highlight code from looking at the source of a web page (Ctrl+U). I rarely, if ever, open a JS file in Firefox; I would be using notepad++ or my IDE for that. This would be about 10 trillion times more useful if it would syntax highlight the source of a web page.
It works and it definitely improves readability, but there's space for improvements... Here are some of the things it misses:
1) It doesn't work with js files accessed from the source view (Ctrl + U --> Click on js file). 2) In my system, line numbers don't show up. 3) In my system, there's an unnecessary 60px padding on the left (maybe related to line numbers not showing up). 4) There doesn't seem to be an easy way to customize the theme.
When the above issues get addressed, it should perfect...