Udacity courses are stuck at the Loading stage when you try to view them with Firefox.
This problem is caused by the HTTPS Everywhere addon for Firefox. Udacity uses YouTube for their videos. And by looking at the messages in the Error Console of Firefox, you can see that there is some problem. It looks like HTTP Youtube is trying to load content from HTTPS Youtube. HTTPS Everywhere does not like this behaviour and thus blocks it.
To be able to load Udacity courses with Firefox, disable the rulesets for all the Youtube entries in the Options of HTTPS Everywhere. On my system, there were two entries: YouTube (partial) and YouTube-nocookie, and I disabled both of them. Once these changes are applied, Udacity courses should load. Restarting Firefox is not needed.
Tried with: Firefox 10.0 and HTTPS Everywhere 2.0.1