Why can't Firefox render certain characters? Instead of certain letters you get a box with a question mark. This issue has been a minor annoyance to me but I could see this being a major adoption blocker for people who are international and trying to use this browser.
It looks like perhaps an encoding problem. I spotted this problem from Hallmark.com.
It's also interesting to note how differently Safari displays fonts using anti-aliasing. Subjectively speaking it looks both bolder and easier to read but readability is tough to gauge without running a readability speed and comprehension test. When I worked at Microsoft we would have heated debates about turning clear-type on or off for Windows XP. Many people claimed that it made the text easier to read while others claimed it made text look blurrier. I'm not aware of any formal testing on this subject.
Either way I'm certain that Firefox has some improvements to readability as a question marks in the middle of a word will certainly slow down comprehension.