I couldn't find any customization that removed window decorations in Linux. I found ones for Windows only.

Also, I got the (subjective) feeling that Firefox was becoming sluggish as I had 6-7 addons. I might be wrong, but that would mean Firefox is sluggish in general.

As I understand it, windows are responsible for drawing their own window decorations on Windows and OS X, but on X they're drawn by the window manager. So it's harder to mess with the title bar because you're starting from nothing and don't even know which of a dozen decoration engines is actually being used. Upside is that decorations are remarkably consistent under X.

Wayland matches the Windows/OS X behavior (probably because it's easier to just accelerate one big rectangular canvas the process draws on), so who knows what will happen as that catches on.

If you can't do it through extensions, you may be able to remove window decorations using the window manager. The configuration is dependent on the specific window manager that you are using, but it should be possible.

In Chrome it Just Works...

Firefox has had years to catch up to Chrome with these issues, why haven't they?

This is great. Mind sharing any extensions used?

Sorry I took a while, but here: App Button Remove the RSS reader in the top-right is Digest The rest is just moving stuff around/removing stuff with the built-in customization reachable by right-clicking blank space at the top. An even more vertical-space saving configuration I use is putting the URL bar up with the tabs - in that case I remove the extra stuff from it like that star with another extension I forget the name of, but it's easy to find.

If you close your tabs via mmb or Ctrl+W, you can remove all those close buttons, too.

browser.tabs.closeButtons;2