I tried all the suggestions in that topic and decided to stay with what I had. My problem is that the screen edges are important because I'm bouncing a ball off the edges (using invisible sprites for the edges) so the screen you see has to be the same as the actual article. Ashley's article is more for games where the edges don't matter and you can scroll.
But that's not a problem for a lot of games and I'm actually happy that the letterboxing works just fine for such a drastic difference in the two phone size/ratios.
And the great part is that Construct 2 exports to the exact format needed to make the Firefox OS App Manager happy! It sounds like what I want (exact knowledge of the screen size before I display anything might just be too difficult at present. What I want is for Construct to read the inner height and width of the screen and adjust the output exactly to those numbers the way I did in my blog with raw HTML. But that's not even close to a showstopper for me.
What I learned by this exercise are three things:
1. Construct is incredibly easy. I hadn't done anything with it in over a year, but it was easy to come back and pick up.
2. Construct is a lot easier than writing HTML5 code!
3. Responsive Web Design for Construct isn't there yet, but the letterboxing is good enough.
Thanks for telling me about the article!