Best way to locate source of Warning: Empty string passed to getElementById()

Was receiving this warning in Firefox.

I just came across this warning and found that there were labels for input fields but the labels for attribute was not set/empty so just completing the for attribute fixed this issue for me.

// Label's for attribute not set which caused the warning in FF

e.g. <label for=''>Text</label><input type="text" name="text" id="text" value="" />

Source File: chrome://browser/content/browser.xul

This means that the error is in a Firefox extension, not in your code.