First problem was working out how to launch the addon. It adds a jigsaw piece icon to the toolbar. It must say somewhere in the docs but I had to google to find out.
Biggest problem is that it says it lists 'Unexpired bookmarks'. I wondered if this is a typo for expired but many it checked and offered to remove were valid. EG:
https://www.facebook.com (TypeError: NetworkError when attempting to fetch resource.) [Remove]
Amazon.co.uk: Low Prices in Electronics, Books, Sports Equipment & more (TypeError: NetworkError when attempting to fetch resource.) [Remove]

Other links were, indeed, broken.
Others users have found it works but not here, I'm afraid. Pity.
I would file a bug report but the support link takes me to Github and I don't know how to work it.

Very good tool for the bookmarks.
The icon is standard and not consistent with the use of extension, please choice an other like a folder or a letter B or something like that.
In addition is very similar to an other extension "Simple Add-on Manager" at https://cup.aihedy.com/en-US/firefox/addon/simple-add-on-manager/?src=api.
Better if black like the official Firefox toolbar icons.
Thanks.

Great-worked well & Fast! Would be great if there were an option to delete all 404's with one click.

Some of the sites are wrongly detected as expired. Can you add a feature to add them to a whitelist/exception list?

Finds problem URLs as advertised but lists as one big flat list. Would be great to have listed in a sortable table, by bookmark folder, error type, URL, age.

Download completes but will not install, says it's corrupt.

(Needs some design though)

How about listing the number of 'expired' bookmarks too?

Ability to filter certain kinds of errors?

Button to 'remove all' expired bookmarks (with a warning dialog of-course)?

Thanks for the addon!

I've used a couple of other dead bookmark finders and 404 has found ones that the others did not.

I do have to note that I've been on the I'net for 20+ years and have migrated my book marks from Prodigy to Netscape to FF.

404 is the best dead bookmark finder I've used. Yes, it is not perfect. But then, who or what is.

Implementation in a panel is not a great idea since it re-checks all bookmarks each time you open it and the results don't cache. The list keeps scrolling to the top as it returns results so you have to wait until it's all done... and forget about viewing another website while it works since you have to start over if you leave the panel.

Cette extension aurait pû remplacer AM-Deadlink avantageusement. Je l'ai installé sur Cyberfox 64 bits dernière version. Au premier clic sur l'icône dans la barre, 404 Book m'a listé un paquet de liens morts sur les 4000 que je dois avoir avec une étiquette "remove" à droite pour chacun des liens morts puis Cyberfox a planté puis si je clique à nouveau je n'ai plus rien à part Cyberfox qui "gèle" plus ou moins. Cette extension est très instable et pas finalisée. Je conserve pour l'instant en évitant son utilisation.

I love what this add-on tries to do but the implementation is very poor. The list of bad bookmarks often disappears for no apparent reason. Plus, there's no way to copy the questionable link (using Cntl-C) so that you can check the site in another tab or browser. If these two things were fixed then the add-on would be way more useful and usable.