As long as the browse button stays optional and doesn't mutate like the firefox "select program" -> I really don't want to have to search for the program instead of just giving its name. That's the reason why I never use the "open with" dialog of Firefox. I don't need to know where my mplayer bin is, since I just start it with "mplayer". That's what the PATH setting is for.

I think most convenient would be to recognize the OS and offer what's best for that OS (selection button for Windows and MacOSX and text field for *nix).

PS and Off-Topic: M$ also created an autoupdater which suddenly says "you must reboot in at most 30 minutes", while you're just working on a new website design... I'm happy with my free operating system (Gentoo GNU/Linux running a KDE desktop). And the forced seperation between user and admin in almost all GNU/Linux distribution *is* better security. Your browser can't just install stuff systemwide or corrupt system services.