The Location Bar and the Search Bar have combined "flex" to fill the space that is available on the Navigation Bar. The horizontal width of both the Location Bar and the Search Bar will automatically shrink as Toolbar buttons are added to the Navigation Toolbar.

Hope this works the same on the Mac OSX platform as it does on Windows & Linux; if you hover the cursor over the space between the Location Bar and the Search Bar a double-arrow <--> will appear. If you 'click'n'drag' to the left or right the width of the Location Bar and the Search Bar can be changed inversely; one is made larger, while the other is made smaller. If that is what you want to do.

Now if you would separate the The Location Bar and the Search Bar by placing a toolbar button (or 'item') between them, that <--> feature would no longer be active.

https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars

Advanced level tweak option: If you just want to make the Location Bar less wide, that can be done by "locking" it to a specific size via userChrrome.css rules or via Stylish.

#urlbar-container {max-width:1050px!important}

Then vary the 1050 number to suit your needs. http://kb.mozillazine.org/UserChrome.css

@the-edmeister

Thanks for you long detailed reply. But I'm afraid I might have not made myself very clear. It is actually in the other direction I want to make the location bar smaller.

Whoops, I would call that vertical or "height".

Been a long time since I fiddled with toolbar height in Firefox, and never on a Mac. Best I can offer is to try out some UserStyles to see what works for you. A quick look at the UserStyles website found this: https://userstyles.org/styles/96122/firefox-slim-toolbar

See also the Classic Theme Restorer extension.

• https://addons.mozilla.org/firefox/addon/classicthemerestorer/