This is how my Firefox looks. Note that this isn't maximized or trimmed, there just isn't a title of menu bar. The menu is accessible via a drop down on the top right as well as window controls.
Here is a list of the addon's and tweaks I used to get this look:
- Chromifox: This is the primary theme used.
- Stylish: For minor CSS tweaks.
- Autohide: This extension allows you to go into Zombie mode and hide the titlebar.
- Compact Menu 2: This extension allows you to hide the main menu and add a toolbar icon instead.
- FaviconizeTab: Lets you minimize tabs to just show their icon.
- When using Autohide you will need to open up about:config and set the setting extensions.autohide.zombieBorder to 0. This will remove the titlebar and border when you go into zombie mode. (Shift-F11)