That is probably because you have one or more bookmarks with a long title.
The width of a column in the Bookmarks menu list is in current Firefox versions automatically set to the length of the longest name of a bookmark, so you need to check the names and make the longer ones shorter.

In a world where everyone is changing the titles of their pages to be complete sentences for SEO, making the bookmarks menu expand to that width is extremely annoying. I'm not going to go and edit all my bookmarks to be short. I'd like the menu to be narrow, as it always used to be.

I am not referring to the bookmark library window, where bookmarks can be edited. I referring to the window that pops up when mousing to the Firefox tools bar and choosing 'Bookmarks', where I then scroll down to the folder containing the bookmark I want to access. It is a real pain to scroll so far to the right to get to the bookmarks. The window used to only be the width of the widest folder name. See attached screen shot.

You will have to check the full list to see if there are any bookmarks in it with a long title and edit the names to make them shorter. Hopefully this will get fixed in a future Firefox version.

I found that a bookmark had not been dropped into the folders in my bookmark window. Yes it had a giant url. But it still an irritating feature that the window will bump to the largest line without any override.

Well the drop down Bookmarks window was OK in FF 4, then to my knowledge it was FF 5, that first had this problem with the over sized drop down window, although I hadn't changed any bookmarks since using FF4...??

So I continued to use FF 4 up until today where I have found out the reason why the BM window was so wide, because of "Long Titles"......?? My questions is now why didn't this affect FF 4, seams to me that they have left out some coding in FF 5 & 6..??

I don't know about the coding differences between Firefox versions. But you can Rename the Bookmarks with long titles to make them short. Here's how you can do it:

-> right-click on a Bookmark -> click Properties -> trim the name of Bookmark -> click Save button

Check and tell if its working.

On a Mac you probably need to do that in the Details at the bottom right in the Bookmarks Manager (Library)

• Bookmarks > Show All Bookmarks

editing the length of bookmarks makes no difference. Dragging the right border of the window does not work. Bookmark window accessed from the menu bar is the correct width. Bookmarks accessed from the navigation toolbar are in the wide window which won't resize. This has only happened in the last few days so may be due to an update. now v 12.0