These preferences will make debugging easier at the expense of lower performance.
The first two need to be toggled, the last two need to be created.

about:config

* javascript.options.showInConsole = true. Logs errors in chrome files to the 
  JavaScript Console.
* javascript.options.strict = true. Enables strict Javascript warnings in the 
  JavaScript Console. Note that since many people have this setting turned off 
  when developing, you will see lots of warnings for problems with their code in 
  addition to warnings for your own extension.
* nglayout.debug.disable_xul_cache = true. Disables the XUL cache so that changes 
  to windows and dialogs do not require a restart. This assumes you're using 
  directories rather than JARs. Changes to XUL overlays will still require 
  reloading of the document overlaid.
* browser.dom.window.dump.enabled = true. Enables the use of the dump() statement 
  to print to the standard console. See window.dump for more info. You can also 
  use nsIConsoleService from privileged script.


This preference will disable the compatibility (version) checking for extensions:

* right click the list and create a boolean extensions.checkCompatibility option,
  then set its value to false and restart the browser.
