In addition, there is a minor bug - if the file name contains the combination of digits 404, the link will be colored red, as if it's not working!

UPD: Works perfect after update. Thank you!

Awesome tool, helps to see the visual indication through the page...Great buddy, you done nice job.

Redirects to 404 pages are considered "valid", can this be fixed?

This add-on is very useful for checking links in HTML pages and other files. It color codes the results for each link. You can tell if a link is broken, slow, insecure or working properly. It is compatible with Firefox Quantum. Very responsive customer service!
Highly recommend.