Normally, as you adjust the width of the browser window, the urlbar and search box will "flex" becoming longer or shorter to accommodate the change. However, they do have a minimum width, and if the window becomes narrower than that, the "overflow" menu/button will appear.

The problem may occur with a wider window on sites with EVSSL certificates, like this one, where the site owner's name appears next to the green padlock. That takes up a lot of space...

You can probably can change Firefox's behavior with a custom style rule, but "something" has to be sacrificed to make space...

Thanks. That doesn't quite solve my problem, because this is happening to me when my window is maximized and there is plenty of whitespace in the urlbar that should get compressed first. But, if the dropdown is a space-saving feature, then maybe something is going on with one of my addons that change the urlbar behavior; if one of them is preventing it from changing size to accommodate buttons on the toolbar, then maybe that's the source of the problem. I'll test.

Edit: Yes, looks like the Location Bar Enhancer addon is the culprit. When it's enabled, narrowing the window size results in buttons being hidden in the dropdown as normal, but when the window is widened again the buttons remain hidden and the urlbar just keeps expanding. Thank you for helping me figure this out.