You NEED one thread for handling user input. Second for painting on the screen.
Than you can have tabs in separate threads. You can have plugins in separate threads. Etc. Lots of use for lots of threads. (On android FF and Chrome work that way).
Yes if C++ is not available to all web browsers on WinP than they are at “same disadvantage”. But they are not. IE is wrote with C++.
Is lazy but if you can not do better, than its best :P Like WebKit on iOS is best cause its ONLY choice.