Try a different ethernet card if you haven't already just to narrow down the source of the problem.
Why would the ethernet card be bunk if literally everything else works fine? I'll sit here, waiting for Firefox to load a page, and I'll browse to it with Opera and it'll load faster than Firefox can even think of doing.
Opera works fine to browse. I think. I do recall one time where I
think it hung... but I don't use it long enough for it to do this.