You may 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 adjusted to fit the bookmark with the longest name.
You can check the names of the bookmarks and make the longer ones shorter.

This feature is useless. I have hundreds of bookmarks, and it will be very time consuming to edit each one. It's not that I chose a lot of long titles: when you save a bookmark initially, it automatically fills in the title (usually with something long.) Earlier versions of Firefox didn't have this problem.

To fix this feature you could do what Chrome does: truncate the bookmark title and add an ellipsis. Firefox is currently unusable to me as it doesn't let me access my bookmarks.

You can try to use the Bookmarks sidebar instead to see if that also has the problem.

• View > Sidebars > Bookmarks

You may be able to set a max-width for the bookmarks if you use the Bookmarks Menu button on the Bookmarks toolbar or the one that you can find in the toolbar palette.

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

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

You can use the ChromEdit Plus or Stylish extension to have easier access to the customization files.

• ChromEdit Plus: http://webdesigns.ms11.net/chromeditp.html

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

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

The Bookmarks sidebar works great! Thanks for the suggestion! But what is all that stuff about Chrome.css?

The file userChrome.css is a customization file that allows to make CSS appearance changes to the user interface.
There is a similar file userContent.css to do the same for web page.

The code that I posted above sets a maximum width for bookmarks item in the popup list of the Bookmarks Menu button. In this case 500px, but you can set any preferred maximum width.
On Windows and Linux you can do this also for the Bookmarks menu, but on Mac that menu is under control of the Mac OS and can't be styled by Firefox.

See also Stylish:

• https://addons.mozilla.org/firefox/addon/2108
• http://userstyles.org/stylish