I check the video, but I didn't find the button, check out screenshot in attachment.
Did you download the Firefox plug-in in this website? http://todoist.com/firefox
Please try reinstalling to see if that helps with the problem. Let me know how it goes.
Yes an that link takes me to https://addons.mozilla.org/en-US/firefox/addon/todoist/ . Reinstall doesn't work.
Can you please check if Firefox is set to accept 3rd party cookies. Also, can you please take a screenshot of the whole screen/browser so I can confirm visually if there's a problem with it?