Great simple viewer Rated 4 out of 5 stars

Once you learn how to work with it, it's great. Nothing awesome, but works nicelly and does what it says it does.
Could have a bar/button to add to the FF window, for ease of use, and the characters are too small by default, but it's great even so.
To open a file, just go to "File > Open CHM Files" or "View > Pannel(?) > CHM Reader".

This review is for a previous version of the add-on (