Because you don't have an input text area focussed, vimium will assume by 'j' you want to scroll down for example, because that's the point of it. Iirc you can

Because you don't have an input text area focussed, vimium will assume by 'j' you want to scroll down for example, because that's the point of it. Iirc you can manually enter insert mode and then it should work fine.