As more and more of your computing happens through the web, and web pages give way to web apps, you may find yourself interacting with web content more than you interact with other parts of your web browser.
When reading or skimming a long page, short toolbars mean you won't have to scroll as much.
Larger buttons are easier to click. More buttons available means you can access more things with a single click.
Unless you started using Firefox very recently, you probably have a "bookmarks toolbar" visible, taking up 21 pixels. If you don't use it heavily, you can...
You can shave off 8 pixels by choosing "Use Small Icons" in the customize-toolbars window. Only the back button becomes significantly smaller.
Remove buttons you don't use and consolidate the remaining buttons onto a single toolbar. (This often works even with third-party addons that have "Toolbar" in their name!)
On Windows 7, a maximized Firefox window gives web pages 32 more pixels than a maximum-height normal window (16 from firefox toolbar rearrangement + 16 from the window border going away).
You can temporarily give the entire screen to a web page by pressing ⌘⇧F or F11 for "full-screen mode".
That's not a question.
You can put your OS dock/taskbar on the left or right side of the screen. You can put your browser tabs on the left. You can even make a vertical toolbar.
Good question! Maybe we should stop letting sites find that out.
I guess it's moderately useful for sites that provide desktop backgrounds or high-resolution videos.
Perhaps a site could detect which of its visitors are saddled with multiple third-party toolbars and offer those visitors a browser that's designed to put users in control. Not that I would advocate doing such a thing.