installed firefox plug-in on firefox 11. Gues what? Doesn't work! Please FIX!
please go to Tools -> Options -> Privacy. Choose "Use custom settings for history" from the drop-down menu and check both "Accept cookies from sites" and "Accept third-party cookies".
If this won't work, clear your browser's cookies and chache.
If this won't help either, try to disable all other plugins you're using, remove the Todoist plugin, re-install it and it it works, enable one plugin at a time to identify which one causes this issue.