It works fine for me in Firefox but your privacy settings may cause Twitter to be blocked.
On that note, I'd like to point out that tweets embedded with in this add-on have DNT enabled and I'm about 60% certain that due to the fact the add-on uses a third party iframe, Twitter does not even know what site they're embedded into.
Heads-up: I'm currently experimenting with alternative ways to compute a tweet's height. If you notice that tweets are the wrong height (either cut-off or too much white space below it) send me a link to a post that displays the tweet and the exact version of your browser.
Edit: actually I've just updated all the iframes so let me know if you notice anything different with tweets, Facebook posts, Reddit comments, etc...