EXCELLENT!
Amazing to transform game server pages, that include config files: Now I can edit code/script with monospace font!

TUTORIAL?
1) Click 3 times the bar icon to refresh the addon; If it doesn't work = keeps "loading", its because that site is forcing its own font types, ex: jsonlint dot com / umod dot org (developer should explain this to avoid 1 stars, not his fault)
To change this behavior, go to Firefox "Options" > "Fonts and Colors" > "Advanced" > On the popup "Fonts", uncheck "Allow pages to choose their own fonts, instead of your selections above". That is it, now it will work. But remember that some sites like Firefox/Mozilla will still block changes.

1) Ok, lets start again: Click 3 times the bar icon to refresh the addon (turn on, off, on)

2) Click the greyed out option "Show more"

3) Choose "Site Font" to change ALL pages on that site domain OR "Page Font" on only remember changing the current page. Like Tod Howard said, it just works!