For those who are looking for a solution for double-left-clicking:
There is a extension called Tab Tweak which supports double-left-clicking close tab in Firefox Quantum, this extension is especially for Chinese Edition Firefox so you might find it difficult to get it from Mozilla's extension library. However, it is available at Firefox China Forum: http://mozilla.com.cn/forum.php?mod=viewthread&tid=319507&highlight=%E6%A0%87%E7%AD%BE%E9%A1%B5%E4%BC%98%E5%8C%96
Click the green button then it will be installed.
I am using Firefox Quantum 57.0 and it works perfectly.