Well, still need to experience before giving any advice

This apparently doesn't works. I am from Russia, sometimes I need English autocorrect tool but unfortunately this doesn't works, after clickng "Add addon" button the English dictionary doesn't appears even after I restart FireFox.

My work is primarily in English, but after installing the latest Firefox Quantum, this language pack was not included. That's fine, I'll just download the language pack, install it, and it should work. Except it doesn't. Tried to check for updates, reinstalled it, but it didn't change anything. I was actually excited to use this browser because as a dev I prefer Quantum over Chrome in terms of performance and UI. But as long as Firefox is unable to do simple tasks like adding new language packs, I'll probably stick with Chrome for now. Too bad...