I haven't experienced any of the issues described by other reviewers and the extension works flawlessly for me. I shall update my review to let you know in case I encounter any bugs. :)

It doesn't seem to add anything at all, nothing whatsoever. I have noidea if this means anything, but I clicked "Debug add-ons", and for this add-on it says "Background script: Stopped". Other add-ons say "Background script: Running"

does not work and shortcut Ctrl + Shift + M is also used by firefox itself

Doesn't work for me and no evidence it has worked for anyone else either :-(
Disappointing.

Not sure how to use it. You can choose a language in the extension's drop down, but when back to the webpage you can only type in the first language (seemingly; I don't have knowledge on the glyph I saw). But most of the time nothing even shows when you type. Ctrl + Shift + M shows the viewport simulator in Firefox, not the functionality claimed in the extension.

1. Tried it on whatsapp web and didn't work.
2. Trying to figure out how it's supposed to work is difficult.
3. Even if you try to select a language you just cannot select it. The moment you close the small dialogue box of the extension the selected language is reset.