I've been using Todoist for a few years, and particularly liked using the Thunderbird plugin. It worked well, so well that it was over a year after Todoist discontinued support for the plugin that I found out that it was discontinued. And it still works, pretty much (only problem I've really noticed is that the automatic sync doesn't seem to work anymore - you have to manually engage the sync) - seems the Thunderbird plugin uses the Web plugin, so when Todoist makes changes to the Web plugin, you also get an upgrade to the Thunderbird plugin.
So, I have continued to use the Todoist Thunderbird plugin. Only problem I ran into was when Thunderbird upgraded to version 60, at which point the plugin would no longer load. That turned out to be easy enough to fix - tweaking the em:maxVersion setting in the install.rdf inside the plugin xpi file allows it to load in Thunderbird 60.
The attached file here is the Todoist Thunderbird xpi file tweaked just that way. I dropped a message to the Todoist Facebook page asking if it was ok if I made this available, and got permission.
Note that this isn't supported by Todoist. They could make a change to their Web plugin tomorrow, and this could quit working, and there's pretty much nothing you could do about it except quit using it. I'm making it available here because I've run into other people who have wanted to try it even if it is no longer officially supported by Todoist.