Want to quickly replace a bookmark? Try out the Replace Bookmark add-on for Firefox. This extension allows you to quickly replace an existing bookmark with a bookmark of the current page.
Here is a little more about the add-on from the developers:
Can be useful for reading and tracking progress in series of articles, webcomics, serial online fiction, etc. When you click on the icon (or use the shortcut), it builds a list of bookmarks from the same domain as the current URL, sorts them by similarity, and gives you a prompt to choose. The new bookmark retains the folder, position, and all tags of the old one. If you check “Keep the old name”, the previous name will also be carried over to the new bookmark.
This is a really neat and simple add-on for Firefox. Pick it up at the Firefox Add-ons website.