Rated 3 out of 5 stars
There should be a separate limit for tabs and windows (e.g. Max Windows: 2, Max Tabs: 20).
A hybrid mode would also be useful, allowing values for both suggest/force modes (e.g. Max Windows: 2/3, Max Tabs: 20/30).
Finally, there should be some way to save pages for later, by allowing to bookmark them easily, perhaps by adding one of the following for each tab listed in the close-tabs dialog: either a dropdown field listing the bookmark folders tree, or adding the empty star bookmarking icon that looks and works just like the one in the location bar.