To determine the default browser from the command line, you can use the command reg query HKEY_CLASSES_ROOT\http\shell\open\command /ve.

C:\>reg query HKEY_CLASSES_ROOT\http\shell\open\command /ve

HKEY_CLASSES_ROOT\http\shell\open\command
    (Default)    REG_SZ    "C:\Program Files\Network\Web\Mozilla Firefox\firefox.exe" -osint -url "%1"

If you just wanted a true or false result for determining whether Firefox is the default browser, you could pipe the output of the reg query command to the find command as below. A result of 0 means "false", i.e., Firefox is not the default browser and a result of 1 means it is the default browser.

C:\>reg query HKEY_CLASSES_ROOT\http\shell\open\command /ve | find /c /i "firefox"
1

Reference:

1. How Does Your Browser Know that It’s Not The Default?
   Date: March 23, 2007
   The New Old Thing | Absurdity in Its Fullest

[/network/web/browser] permanent link