The login which determine if we should attempt to fill a login form with a saved login all lives in LoginManagerChild.jsm, in a method call _fillForm. Its a long series of decisions and checks, nested ifs etc. that sorely needs refactoring to make the logic clearer.

The function complexity guardrails managed by eslint are currently disabled for this function, and over time it has continued to grow - currently it has a complexity of 74; the (generous) threshold we enforce is 59.