Awesome project. I'm not sure if it's just in Firefox, but I would recommend that you be a little more aggressive with the whitespace in the books.
I've found that iBooks has a great display stylesheet for epub and what makes it great is the ratio of text size to whitespace. It also limits how wide text can get - no need to spread it across the whole page. You could probably double the size of your margins to improve this.
I am also not sure if Firefox is causing this, but inter-line distances do not seem to be consistent and that is very distracting as a reader.
In both browsers the next/prev page arrows are also weirdly off-center.
I've found that iBooks has a great display stylesheet for epub and what makes it great is the ratio of text size to whitespace. It also limits how wide text can get - no need to spread it across the whole page. You could probably double the size of your margins to improve this.
I am also not sure if Firefox is causing this, but inter-line distances do not seem to be consistent and that is very distracting as a reader.
In both browsers the next/prev page arrows are also weirdly off-center.