Finally found the best replacement for the old addons that didn't work anymore with the new Firefox webextensions. it gets the job done without any hassle.

The only minus point so far is the design of the pop-ups window, since it's basically not designed as a pop-ups window to begin with, it's highly inefficient, too much wasted space, not adaptable to the position of the selected word. Especially when translating word that are in the bottom of a web page.

I don't know how much works the developer needs to design a new pop-ups design rather then just using whatever google provide, but one could only hope.