Had no idea if it was connecting to MAL correctly or not, it just always showed the login prompt. So I tried re-entering the password, disabling things like noscript, and similar but then got locked out of my account due to too many failed logins.

Needs some screenshots of the add-on itself (the prompt window) and, if it doesn't already have one, a login success notification so you know it worked. Maybe have it pop up a little message or a # next to the icon when it successfully detects and updates MAL too.

Love it. Been wanting something just like this for a long time, and couldn't be happier with this one. I'm a mess when it comes to reading manga and end up with 15+ tabs of all the ones I'm trying to catch back up on, and this just helps so much it's unbelievable.
**Troubleshooting tip blow**

if you're a Umatrix user, make sure you add the rule "* myanimelist.net * allow" in your "My rules" tab. Or if you're paranoid you can be more specific and just do "* myanimelist.net xhr allow"