Firefox for Android has no toolbar popup, so the settings page is the whole extension there. It wasn't built for a phone. Now it is.

Fixed
The settings page rendered at desktop width on Android, scaled down to something close to unreadable. It was missing a viewport declaration, so the browser assumed a ~980px page and shrank it to fit. It now sizes to the screen, which also means the responsive layout actually engages.
The emoji picker's full-screen mode on small screens never triggered on a phone, for the same reason.
Changed
Below 620px a rule stacks onto three lines — the word with its on/off switch, the replacement below it, then the two match options — instead of squeezing seven columns onto a 360px screen.
The Aa and Whole word checkboxes carry captions in that layout, since the column headings they belong to are hidden.
Everything tappable is at least 44px, and text fields are 16px, which stops Firefox zooming in every time one takes focus.
The add-a-rule form goes one field per line with a full-width button.
Below 380px the delete button drops to its own line so the word gets the full width.