A button that replaces the bookmark location that shares the most characters with the current tab with the current tab's location.
For those of us that are way behind on your favorite web comic, the only way to make any progress is to:
1. Open a bookmark to the latest comic you've read
2. Read any number of comics
3. Manually copy and paste the new link to the latest comic you've read back in to your bookmark
This add-on automates that process by providing you a one click bookmark updater. It works by finding the bookmark whose link is closest to your current tab (ie, the closest link to your current tab of http://xkcd.com/699/ might be your bookmark to http://xkcd.com/680) and replacing that bookmark's link with your current tab's link. In the case where there are two bookmarks that are equally close, you'll get a popup saying that the add-on could not find a unique closest bookmark and no changes will be made.
Note this extension is also available for Firefox: https://addons.mozilla.org/en-US/firefox/addon/comicupdater/