qBittorrent official forums

Ok, I know this problem has been around off and on for a while.
When logging into the QBT UI from Firefox the login screen keepts replying with "Invalid Username or Password"
Can log in fine using Chrome or Dolphin browsers, and connects fine from the android Controller app.
Could it be FF not accepting a self signed certificate?

it should not be problematic, but your description let me think more about caching issue.

Try with FF in private mode. If not working clear cache and credential for the concerned site.