I like the idea of it, however, when dealing with several 1,000 bookmarks and lets say a 1,000 are a mixture of 404 or possible 404 its to time consuming to individually select each one so not to include the possible 404.

Maybe group them better (e.g. redirects, timeouts, resource not found, unspecified 40x/50x, server not found, etc.). Allow one to check mark one box scroll down holding shift and click further down and will select everything in that range.

UPDATE 20190911: I decided to start with a fresh profile. In it is a total of 9 bookmarks; 5 of which are the default bookmark preloaded the other are 4 I added. When I run 404 nothing comes back under expired or other which is correct. However, I noticed it reports only 4 unexpired bookmarks rather than 9. So is it intentional to exclude the preloaded bookmarks? NOTE: No Adblockers or any other extension other than 404.

Useful, but the add-on would be much better, if it was possible to check, if wayback machine has a version of the dead site and to replace the original link with the link to that stored version at wayback machine.

It appears to function perfectly, but inconveniently. It would be much more convenient to use if the remove link was first on the line next to the link to the website. that way i could hover over the link, see where it points, move slightly left, and click remove rather than try to move my cursor all the way along the text to the opposit side of the window to click remove

The tool is showing a lot of valid links with a status:
(TypeError: NetworkError when attempting to fetch resource.)
Not sure if this is due to a too short timeout or maybe some other status codes (like redirects)... but is making using the tool time consuming, as I still need to manually validate it's assessments (OK it's still better than going through all of my links manually).

There shall be an option to filter the list based on the status code (many 401s (Require Authentication) are valid links, so shall be a way to exclude them from the list).

Also, I have quite a few bookmarklets - the address actually containing javascript (starts with javascript:) - they shall be omitted from the list.
Keep up to great work, thanks a lot for your efforts to make our life easier ;-)

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.