I buy about a dozen items per year on eBay, and when I'm bidding on an item, I add its bookmark to my personal toolbar, then I click that bookmark repeatedly for the duration of the auction just to find out two simple pieces of information: the current bid and the time left.
Instead of making me click the link over and over to get that information, Firefox should automatically check the auction periodically on my behalf and add its status to the bookmark label. Then I could keep track of the auction just by glancing at my personal toolbar.
Auctions may be my personal itch, but I reckon there are other auctioneers with the same itch, and a feature like this could be handy elsewhere, too. A bookmark to an Amazon product could display the product's current price, for example, while a bookmark to a news site could show its latest headline. Basically, any page with important and regularly-updated but short summary information is a candidate for this feature.
It might even be useful to display these summaries in other places--like tabs and window title bars--where the browser currently displays page titles. And it might make sense to define a simple microformat to let the pages themselves specify which of their content represents summary information and how often to update it, although we'd want to allow users to override that where appropriate, just as we let them choose their own bookmark labels.
This feature extends a concept that live bookmarks introduced: that bookmarks can display much more than just static links to locations, they can display regularly-updated summaries of those sites. So I'll call this "son of live bookmarks," for lack of a better name, and in homage to the same cheesy horror flicks from which Mozilla derived its name.
Would you use this feature if it was available? Can you think of other kinds of pages for which the feature would come in handy? Got a better name for it? Comments welcome.