Strange, I thought that was a well known item in FF
Oh well, you do the same in IE7 by the way, just open the tabs you want, select tools, Internet Options, General tab, Use current.
In IE7 though, you now have a little drop down list by the homepage Icon that you can use to select a single page from your homepage list if you don't want the lot opening. You can also use the dropdown list to edit your homepage list easily and quickly.