Right-click on the bookmark, choose Properties. In the small Properties window, the first space is the "Name" (you can change that), the second space is the URL (change that and you will not be able to go to that page).
You need to check your Plugins and update as needed.
Excellent precise answer. Wish the four community answers to a similar question was as precise. I could make no sense out of them. I can't believe Firefox does not have Help Index like Windows has. It would have saved an hour of my time! I guess you can guess what I think of think of
Firefoxe's Help menu.
When the Help was built into Firefox, it seemed to always be be out of date. So Firefox did what many other open source software providers had already done, they switched to online Help articles. IMHO, after doing Firefox support for that last 10 years or so, the built-in Help file was so underused by the users who needed simple answers, that it was a waste of scarce resources to keep it up to date.