How to Check Status of Links in Bookmarks

This seems to be a pretty good extension for Firefox that does what you want.

It adds a small icon between the URL field and the search field in firefox. When you click it you can initiate a scan of all your bookmarks.

Description from addons.mozilla

Bookmarks Checker. Scan favorites for broken or dead links. Delete empty, and duplicate folders.

Check bookmarks for broken or dead/broken links.
Add bookmarks that you do not want to be scanned to ignore list.

Scans for 404 bookmarks, Time-out links or 403 favorites.

Scan for duplicate folders.
Scan for empty folders.
Scan in several threads.
Set timeout setting for bookmarks