The Manage Folders add-on for Firefox places a “Manage Folder” item on the context menu of bookmark folders, which opens the “Places Organizer” expanded to that folder.
Here is more information about that add-on from the developers:
Firefox plug-in which places a “Manage Folder” item on the context menu of bookmark folders. This opens the “Places Organizer” with the specific folder expanded, ready for editing. This feature, originally in Firefox 1.0 but removed in 1.5, makes it easy to organize a specific bookmark folder when you realize it needs to be re-organized.
Want to learn more? You can get more information about Manage Folders from the Firefox Add-ons website.