Actually this works for me on Windows 7 both in 56 and 57, and without any add-on. However, that behavior depends on whether or not the Title Bar is displayed; if you don’t have that enabled, clicking the tab bar increases or decreases the window size as apparently the space clicked is considered to be part of the space above it. If you do have it enabled (use the menu button > Customize > Title Bar at the bottom left), this is default behavior. This is the same as toggling the ''browser.tabs.drawInTitlebar'' preference in about:config. If you do not want the title bar to be displayed and still be able to double-click to open a new tab, see if an alternative add-on for yours is offered on on www.arewewebextensionsyet.com or https://mozilla.github.io/extension-finder/.