Excelent addon, I use this even before the webextension version.

Only 2 things I think could be back:

1 - This I have tiny idea that you can't do with webextensions, since they seems a bit more limited. But suggest doesn't hurt:
Shortcuts to open the translate window. I've never opened this window by other way before the webextension version...

2 - This one I have literally no idea if is possible or not with webextensions, but it'd be good to have again:
Set the focus on the textbox of the translate window when open this (by clicking in the icon or by shortcuts)...