Debugging a Firefox addon is a slow process: (1) edit source code in a JS editor (2) package into XPI using a build script (3) drag into Firefox to install (4) restart Firefox (5) open the JavaScript Debugger
Can we speeden up the process? Like install it into Firefox without a restart, or configure the build script to install it into Firefox as well?
You'll want to locate your profile folder. Once you find it, go into the folder called 'extensions', and then locate the folder for the add-on you are working on. Replace that folder with a file of the same name (minus .xpi if that's part of the name), and inside the file place the full path to your source directory of the add-on.
In Linux and Mac, that'll look like:
/full/path/to/yourExtension/
And on Windows:
C:\full\path\to\yourExtension\
Remember the trailing slash and beware of extra whitespace.
As long as you don't use jar files inside your add-on, you will no longer have to rebuild (this is covered in a bit more depth here).
Additionally, you'll want to set nglayout.debug.disable_xul_cache to true. For edits to xul or js files, you'll just have to open up a new window to see your changes instead of restarting the application. There are other preferences here that you may find useful as well.
Extra tips:
Starting firefox with a certain profile (dev), while other firefox profile is open and with the debugger already on:
"C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -no-remote -P dev -jsconsole
Check out the restart addon (saves time).