Before starting the tutorials, take some time to learn what's available in the Firefox developer tools. You won't need most of them at first, but knowing what's available will help as you gain experience.
I haven't taken any but Udacity has some javascript courses (https://www.udacity.com/courses/all), just tick the "Javascript" checkbox under "Technology" in the left menu
MDN Javascript resources: https://developer.mozilla.org/en-US/docs/Web/JavaScript
Before starting the tutorials, take some time to learn what's available in the Firefox developer tools. You won't need most of them at first, but knowing what's available will help as you gain experience.
The two developer tools you'll probably use most when leaning are the Scratchpad (https://developer.mozilla.org/en-US/docs/Tools/Scratchpad) and Console (https://developer.mozilla.org/en-US/docs/Tools/Web_Console)
https://jsfiddle.net/ & http://jshint.com/ will also be very useful while learning JS
I haven't taken any but Udacity has some javascript courses (https://www.udacity.com/courses/all), just tick the "Javascript" checkbox under "Technology" in the left menu
Some additional resources:
https://developer.chrome.com/devtools
https://msdn.microsoft.com/library/bg182326(v=vs.85)
https://msdn.microsoft.com/en-us/library/d1et7k7c(v=vs.94).a...
http://www.ecma-international.org/ecma-262/6.0/index.html
https://github.com/getify/You-Dont-Know-JS
https://addyosmani.com/resources/essentialjsdesignpatterns/b...
Dated but still worth watching:
https://www.youtube.com/watch?v=hQVTIJBZook
https://www.youtube.com/watch?v=bo36MrBfTk4
edit: added addl. resources