That can be caused by having some bookmarks with a long title in the main Bookmarks menu.
The width of a column in the Bookmarks menu list is in current Firefox versions automatically adjusted to fit the bookmark with the longest name.
You can check the names of the bookmarks and make the longer ones shorter.

I was hoping that wasn't going to be the answer because I have over 100 bookmarks. Guess I'll have to live with inconvenience.

You can use the Bookmarks side bar (View > Sidebar > Bookmarks)

It should be possible to set a maximum width of bookmark items that show in the Bookmarks Menu button drop down list, but on a Mac you can't do that for the Bookmarks menu.

Add code to userChrome.css below the default @namespace line.

• http://kb.mozillazine.org/userChrome.css

The customization files userChrome.css (interface) and userContent.css (websites) are located in the chrome folder in the user profile folder.

• http://kb.mozillazine.org/Editing_configuration

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* only needed once */

#BMB_bookmarksPopup .bookmark-item { max-width:500px !important; }