It would be great if it was possible to rename bookmarks directly in the bookmarks sidebar (that shows up on the left, e.g., clicking cmd+B on a Mac). For now the only option to change the name of a bookmark or bookmark folder is to right-click > Properties > enter name > click Save. Many other programs (for example the Finder) let you click a second time on an already selected item and that lets you edit its name. Another option would be to use a shortcut (such as fn+return or alt+return etc.) and it’d be great if one of these options would be added to Firefox.
I was looking for an add-on that might do this but afaik none of the add-ons currently available has that functionality – if so, please let me know 
Thanks you for considering this!