> Would it be possible to make it possible to have the information on a single line as was the case before with the addon bar ?

With the settings you can limit the display of days, nights, icons, labels

> Is theren't on this browser an addon bar similar to the one we had on Firefox?

I do not know.
The current version of Forecastfox uses the same code for Firefox and Chrome (Opera is also based on the Chrome engine)

PS: Please use the forum for discussion and questions:
http://forums.mozillazine.org/viewtopic.php?t=2913287