I hope you have bookmark collections.
Bookmarks are efficient, save energy, and are more private, than using “Google” for everything all the time.
But my collections over time include tons of websites that are down!
This is quite sad, but at least using that addon you can filter out those websites.
Or you could search for the link on archive.org, archive.is or other archiving websites.
If the content on a site is important enough for me to bookmark it, I’ll likely make a local copy for exactly that reason. I want to be able to go back to this information whenever I want, with or without internet access
Would be awesome to have that neatlessly integrated into Firefox. I know SingleFile, but auto detecting local backups would be lit