Edit: After re-reading your comment now I realized that I had probably misunderstood it. It seems to me that you request a functionality to remove "dead" bookmarks.

As I had already replied to a previous comment requesting this, I had already played with such checks. But there are so much inherent difficulties (URLs responding in nonstandard ways, temporarily delayed, replying only when certain context strings are sent, etc.) that this would require a lot of fine-grained configuration options to work properly, and it would completely shift the focus of the extension. Also, it would require permissions which IMHO are not appropriate for the extension.

There are other extensions specialized only on such checking. I recommend to use these instead.