If you have the default settings, when you open a link from an external application, it should become a new tab in the window that was active last. Does that work? If so, try the steps I gave in these threads:

• Can you have a different action when you open your browser to when you click the home button in the browser?

• How do I open 2 websites on startup and 1 website when clicking the home button?

See also:

• https://developer.mozilla.org/Command_Line_Options

Note that if you want to open a lot of website, you may run out of space and exceed the allowed maximum for the target field. If you exceed the maximum length of the target field then use a CMD file (.cmd) with the start command.

start "" "<path_to_firefox\firefox.exe>" URL1 URL2 URL3

• http://technet.microsoft.com/en-us/library/bb491005.aspx