Thanks for the feedback. I've read in various places that it's bad form to embed fonts, but in some cases it's unavoidable. My book uses phonetic characters and other odd things that aren't found on all systems. ADE in particular is infamous for its extremely limited character set: it won't even display Eastern European languages properly. It's often pointed out that Math and science texts need embedded fonts.
I can't help wondering where the recommendation that epubs use .otf fonts comes from. I've seen it in the Adobe specs, and that's no surprise, since that's what Adobe sells; but my TrueType font works well in ADE and (mostly) Calibre. Does anyone know why it's so often stated that .otf fonts are preferable in epubs? i don't get it.
I tried various directory arrangements while working with this problem, on the theory that EPubReader or Firefox was being picky about their locations: nothing made any difference. And it seems pretty well established now that there's a problem in the CSS that ships with EPubReader.
Finally I'll just say that if our excellent developer wants any help testing a fix, I'll be more than glad to help!