I just found what looks to be a rather decent EPUB reader add-on for Firefox, EPUBReader. I’ve installed it on my desktop already and tested some EPUBs and at first blush it looks like a decent reading experience. I don’t mind checking my EPUBs in Calibre with the default E-Book Viewer, but this looks better IMO for casual reading.
I’m also in the middle of adding Firefox mobile to my N900 (again) and EPUBReader as well. Which incidentally is how I found it – searching for possible alternatives to FBReader for Maemo.