Duolingo is not fully loading in firefox 27.0
I just get a white screen with support on one side and the bottom text menu on bottom. The bottom menu does not always appear either if you reload.
This solves most of the problem, but some links on Duolingo automatically open in a new tab with regular HTTP so the page fails to load. If I manually add the HTTPS to the URL in the new tab it often loads properly, but not always.
For example, I went to the "Guide to Finding Duolingo Guides" (https://www.duolingo.com/comment/1278938) and clicked on links to http://www.duolingo.com/comment/1480196 and http://www.duolingo.com/comment/1076946 which both opened in new tabs and failed to load. Manually changing the first one to HTTPS made it load, but that didn't work for the second one.
I'm using Firefox 26 with HTTP Everywhere 3.4.5 on Linux Mint.
Yeah, forum post links go wherever the post author decided they should go. In particular, they use HTTP if the provided link starts with http://. I'd write a HTTPS Everywhere rule to automatically rewrite HTTP links to HTTPS ones, but I think Duolingo devs want to fix it on their side as well (because other extensions may also be affected, for example).
As for the last link: You can't see the post even if HTTPS Everywhere is disabled, so I'd guess it was just deleted... Usagiboy7 was the post author and he said "I forgot and deleted the old one first" in the guide you mentioned, so it's possible that he deleted it himself.
If anyone is still having this problem the solution is simple. You DO NOT need to disable HTTPS Everywhere. Just ALLOW Cloudfront from the filter list (you can do this from the filter list or toggle the Cloudfront from the HTTPS Everywhere icon). Cloudfront is Amazon's CDN and is probably used by Duolingo now on the back-end. You should also set the Duolingo url to https://www.duolingo.com/ then the default filter for Cloudfront usually works.