A good idea and a useful tool, Didier, thankyou!
I have only rated it a 3 star because since the Firefox upgrades (I am on version 60.1 currently as of August/4/2018) because its not always working with a lot of websites; and also because it has been well over a year since you last updated!
NOTE:
To all of the people past, present and future who use this add-on...
From what I have read in the comments, and the lack of any replys from Didier, I IMHO think that the author ...
1) Can't be bothered to interact...for any number of reasons
2) Has lost interest in keeping it updated (possibly due to Firefox updating so often etc.)
3) The author is having health problems and can't continue working on this add-on
4) The author could have died (a point a great many people don't even think of)