Places’ Full Titles is a tiny extension for Firefox that eliminates unnecessary truncation of bookmarks' and history items' titles.
Make “Smart Bookmar...” a thing of the past! ...or something.™
To leave space for sub-menus, menus that unfold vertically still have a size limit, but it's somewhat wider than usual. This applies to the top-level Bookmarks and History menus, to any folder on the bookmarks toolbar, and to the bookmarks toolbar's overflow menu.
It's an updated, slightly-improved and renamed version of Bookmarks' Full Titles for previous versions of Firefox. (If you've already got Bookmarks' Full Titles, it'll be upgraded automatically.)
You can get it from Mozilla Add-ons.