Android is not like iOS or Windows for mobile. Default apps are handled much like on Windows, even down to the home screen.
If you install a new browser, for example, then it will give you the option to change the default the next time that you try to open any web link. Same goes for most anything. If you install a new app launcher (home screen) the next time you press the home button it will prompt you for which app launcher you want to use (since they're just normal apps).
Anything that opens "web pages" automatically uses the current app for opening web pages which can be FF. If you install FF it will prompt you and you can tap FF and then tap "always" to skip the prompts.
Many things like Facebook and Google search will initially use their own internal browsers for speed, unless you configure them not to. Even in an internal browser, you can generally reopen the page in FF.
