After receiving some positive feedback on my "son of live bookmarks" idea, I wrote up a more comprehensive description and proposal for a Firefox feature that supports the display and updating of microsummaries of web page content (tip o' the hat to Mike Shaver for the term "microsummaries").
And then, since it's easier to understand a proposal with some working sample code, I hacked up a prototype in the form of an extension you can install into a Places-enabled build (milestone 2 or newer).
The prototype comes with built-in support for three kinds of microsummaries: eBay auction items, Yahoo! Finance stock quotes, and Merriam-Webster's Word of the Day. Just throw a link to one of those pages onto your bookmarks toolbar, and the prototype will start updating it regularly with pertinent information (the price/time left in the auction, stock symbol/price, and word of the day, respectively).
Thanks to Brian Slesinsky, who graciously trilicensed XPath Checker code, the prototype also supports user-defined microsummaries. Just context-click on some text in a page, then select "Watch [the text]" from the context menu. The prototype will add a bookmark to your bookmarks toolbar whose title is the text you clicked on, then it'll update it regularly.
Here's a screenshot which demonstrates microsummaries for the three built-in types plus a user-defined microsummary (the number of lines changed in a Bonsai query for "Places checkins in the last day"):
Note that this is an early prototype. It has limited functionality, lots of bugs, and relies on the rapidly evolving Places code (so is susceptible to bustage). Don't rely on it or expect any eventual Firefox feature to work like it. It's just a proof of concept.
Also note that it'll take up to 15 seconds for a bookmark you add to the toolbar to start showing a microsummary. After that, the extension will update the microsummary every 30 minutes.
Finally, note that Places mucks with your profile, migrating history and bookmarks to new databases. Make sure you know what you're doing (or are using a fresh profile) when you try out a Places-enabled build.
If I haven't scared you away yet, then give it a try, and let me know what you think.