If you set a master password for saved logins in Firefox then passwords won't be available with a simple click, and they will be encrypted on disk.
In browsers that use the OS' password storage then they will normally be stored in encrypted form, although the browser integration is seamless so you won't notice the difference.
In both cases, there is a significant security advantage in cases where the data on disk is leaked (say, if someone steals your computer and you don't have full-disk encryption.
Firefox uses a weaker scheme, but the passwords are still encrypted and it's definitely less accessible for an intruder compared to a plain text file.