Do you mean editing a bookmark in the Bookmarks Manager (Library) ?

For a bookmark on the Bookmarks Toolbar or in the sidebar you can use "Edit Bookmark" in the right-click context menu.

• https://support.mozilla.org/en-US/kb/bookmarks-firefox

Have you read this page yet: https://support.mozilla.org/en-US/kb/how-add-remove-edit-and-move-bookmarks-firefox-ios

This article is for users using Firefox on iPhone or iPad as iOS is Apple's mobile OS. The OP is using desktop Firefox on macOS. When you use the search make sure Firefox is selected on the right so you only get articles for desktop Firefox.