I have used offline English dictionary for years with other browsers, always when I long press a word it pops up an option for dictionary definition. New phone, new Firefox, no dictionary. Have the same dictionary app installed, and tested working on Chrome. Hate Chrome. No add on from Mozilla add-on site works, have tried several now. I expect this is because they're all designed for non-Android but there is no way to just search for Android compatible add ons. Utter crap.