The advice from Duncan, here is still true:
https://addons.mozilla.org/en-US/firefox/addon/todoist/reviews/811768/

In addition to following these steps:
https://support.todoist.com/hc/en-us/articles/208951085

The tl;dr is:

Under Settings - Privacy - History - Exceptions, add and set ALL of the following (not just the http like the doc says) to "Allow"

http://todoist.com
https://todoist.com
http://cloudfront.net
https://cloudfront.net

Also be sure to add todoist.com to your exceptions for third-party cookie/history managers, and make sure that if you're using "Clear history when Firefox closes" that you're not also clearing your site preferences. Because, well, then you lose your site preferences, like all those handy third-party cookie exceptions you just went to the trouble of adding.