If you want certain webpages in tabs, while others in a new browser window just load Firefox with parameters:
E.g. to launch a certain webpage in a new window make a keyword an insert somthing like this in "Target:":
"c:\program files\mozilla firefox\firefox.exe" -new-window http://www.google.com
For more info on parameters:
http://www.firefox-browser.de/wiki/ParameterGues somthing similar can be done IE.
Regards R