Using notepad++
Under the menu Run there should be an option for 'Launch in Firefox' Keyboard shortcut is CTRL+ALT+SHIFT+X
In Firefox version 35.0, press F12. It brings up the inspector, but there are other tabs. Console (javascript console),
Debugger, Style Editor (for editing CSS format style on a webpage), Performance (webpage profiler, can be used to profile Javascript code), Network (details network operations of webpage)
However, as you're content with Firefox, I should note that Mozilla recently launched Firefox Developer Edition.
https://www.mozilla.org/en-US/firefox/developer/
edit: the keyboard shortcut in regular firefox to bring up the debugger is CTRL-SHIFT-S
Also, firefox includes a WebIDE but it's specifically designed for making Firefox OS apps. Those are written in HTML5/CSS/Javascript. So you could use it for your javascript experiments if you so choose. Default keyboard shortcut to launch WebIDE in Firefox version 34 and up, is SHIFT-F8
Oh, and, I don't remember if it's there by default. But there is a menu icon for pulling up the developer menu in firefox. If you click the menu icon (3 vertically stacked bars) and don't see a wrench labled 'Developer', click the button at the bottom of that menu that says 'Customize' Then drag the wrench icon over to the menu.