It's somewhat of a taboo around here, and every time I have mentioned this there has been a bunch of responses certifying that Firerox works perfectly for them.
I genuinely can't think of any sites I come across that are broken, at least visibly enough for me to notice. I think that speaks more to the variety in browsing habits than anything else. I'm sure they exist and I don't think it's a taboo. People who don't share that impression probably just don't visit any of those broken sites, e.g. me.