browser.i18n.getMessage( messageName, // string substitutions // optional any ) https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/i18n/getMessage

Saw this after just posting my own question: https://developer.apple.com/forums/thread/698319 The html tag has the right height but whatever that's holding the html tag is not adjusting its height accordingly. This is only problematic for iPad because iOS shows popup in a different way. Sorry should have added a comment, not answer. Don't see an option to remove it.

I'm able to load Safari extension app on another Mac logged in by a different user who is not enrolled to Apple developer program. I suspect the extension app just needs to be signed (I have signing automatically managed with team set). I just choose "copy app" option when distributing it in Archives window (Window->Organizer).