I think the last version with a flexible space was Firefox 28.

Were you or are you using the Classic Theme Restorer extension to bring back the flexible space and other toolbar controls?

If you haven't tried it yet, be aware that CTR makes some major changes when you install it, but these can be controlled in its Options dialog. To access an extension's Options dialog, you can use the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then find the extension and look for an Options button.

There is a simpler one that has what you need, but I haven't about it.

Thanks. I recently completely removed Classic Theme Restorer. First I had disabled it. I also restored defaults in customize windows. I don't think disabling it is what removed the flexible space. I don't remember. I think removing the addon did it.

I had originally added Classic Theme Restorer to get the bookmarks sidebar button. I have since found View Sidebar to get such a single-click sidebar button for bookmarks. The weird thing is that the original sidebar button (the star button) sometimes shows up. So I think Classic Theme Restorer may have left some flaky remnants after I removed it.

I have been unhappy with Firefox lately. So many things changed, and not for the better. I don't understand why the developers could not have left a flexible space in the customize window. And a single-click button for sidebar bookmarks.