I tried "intervals" with almost the same configuration as you (Opera 10.51, Firefox 3.6, 1.9.7) and it worked for me, both in Opera and Firefox. My Flash player is 10.0.45.2 - you could maybe try upgrading your flash to the latest version, although I doubt that this would change anything since Flash isn't loading at all on your pages.
). Restart Firefox.
4) Load a question page in Moodle - after loading the page, go back to the debugger window and check if any error message appears in the console (black window)