Firefox comparatively good at storing passwords securely
If you use a good master password. Google Chrome and Internet Explorer were worse at it on Windows, according to the below linked article:
However, it would be unfair to end the post saying that browsers are completely unreliable at storing passwords. For example, in the case of Firefox, if a strong Master Password is chosen, account details are very unlikely to be harvested.
Read more: Link - RaiderSec: How Browsers Store Your Passwords (and Why You Shouldn't Let Them)