they don't man!!
ok here's a tip the browsers installed on your pc,can be found in the registry of ur pc ,use run->regedit->HKEY_CLASSES_ROOT \ http \ shell \ open \ command
u'll c if ur using firefox as default browser: c:\......\firefox.exe....
for IE you'll get something else
so try working on this if ur still insiting on using the shell command
but i'm sure there's another way to do the trick and much easier guys, but i have no time to wrok on it right now.
so use the tip that i just gave to determine what's the default browser.