Check Places is a Firefox addon that checks your bookmarks for offline webpages and duplicates. Once it checks all the bookmarks (which takes little time), you can delete all the broken bookmarks, or choose to delete them selectively.

CheckPlaces is an experimental addon, so you have to tick a checkbox and install it. However, it worked flawlessly for me on Firefox 3, so the experimental status should not really be a worry.

Check Places gives you options to check for offline bookmarks, duplicates or both. To avoid the addon hanging up for a long time when checking for offline pages, you can set a timeout – the duration depending on your network speed and latency. The default, 60 seconds, should be large enough for all.
If you have bookmarked duplicates intentionally, you can tell Check Places to exclude folders while checking. Alternatively, you can check only the specified folders.
Download Check Places Firefox extension
Cool addon