> Would there be some sort of uproar if people found out that Firefox had special optimizations for common websites, for instance?

There was a comment on this site a few days ago that claimed Google was being anti-competitive by hard-coding a list of their web properties into Chrome for it to send diagnostic info when visiting them, so yes, anything where there's a special path for some 1st/3rd parties but not others could be construed in many ways that look bad for the author.