The Jez McKinley Wiki

How to restore the double-click for new tab functionality in Firefox

When you install Tree Style Tabs in Firefox, you lose the ability to double click the tab bar to get a new tab. This is because the tabs are no longer 'on top' and is annoying to me. One option is to install Tab Mix Plus, but that is a big extension for a single feature.

The alternative:

mkdir ~/.mozilla/firefox/<profile folder>/chrome
vi ~/.mozilla/firefox/<profile folder>/chrome/userChrome.css

Add the following content:

#navigator-toolbox:not([tabsontop="true"]) > #TabsToolbar {
  -moz-binding: url("chrome://global/content/bindings/toolbar.xml#toolbar")
!important;
}

or

#navigator-toolbox > #TabsToolbar {
  -moz-binding: url("chrome://global/content/bindings/toolbar.xml#toolbar")
!important;
}

(taken from here: https://bugzilla.mozilla.org/show_bug.cgi?id=575248#c27 )