I'm currently running Firefox 22.0 on Windows 8, and I can't get my Todoist Firefox addon to work.
The sidebar opens, and I get the loading animation, but it never loads.
Please go to Firefox's Options -> Advanced -> Network and remove todoist.com from the list at the bottom, then in "Privacy" select "remove individual cookies" and delete the todoist cookies, reload the browser and let me know if the issue persists.
Also, in the "Privacy" settings, choose "custom settings for history" and make sure that third-party cookies are accepted.
Thanks so much for those instructions; it's working now! :)