I'm extremely happy my favourite extension became now Web-Extension. I only use 3 addons ( Google Translator, Foxy Gestures and Ublock Origin ) while 2 of these are Web-compatible. Only Ublock Origin is not yet.

By the way, I'm using Fx 55 beta 1 and Google Translator 0.54, but the bubble doesn't show up after I highlight a word. Currently in order to avoid this, I'm relying on an other combination ( Press Shift + Highlight ) to do the job. I'm pretty sure it will be fixed...