From mircs help file:
/url [on | off | show | hide | -dranils] [[N | mark ] | address]
Show or hides the URL list window, and allows you to modify the current list of addresses in it.
The -r switch deletes the Nth item, or all items that match the mark you specify.
The -an switches allow you to open a browser window to an address, where -a = activate browser, and -n = use a new browser window.
The -ils switches allow you to insert an item, load, and save the list, respectively
/url -n http://www.yahoo.com
would open the default browser in a new window to yahoo.com . Using just "/url http://www.yahoo.com" will use the open browser window and open a new tab in it to yahoo.com .
I don't know the advantages of using /url over /run other than being able to decide where to open it, in a new or the active browser window.