It should be possible to change the date and time format separate from language, especially for US English.
Firefox is a platform on which a huge number of other applications run on top off. Many or most of these applications depend on Firefox to render dates and times in a format suitable for the user.
At the same time, many users have their os, browser or application settings set to (US) English, even if they are not from the US. There are many reasons for this, for example that the application has a poor translation to their language, they're living or working in an internationa environment, or they just prefer it. It's reasonable to assume that many applications are either made in English or has English as one of their first languages if they are internationalized.
In fact - English is a language with more, dramatically more, speakers than native speakers. More than a billion people speak English as a second language, and a significant fraction of these may want to use English in various applications while not having the rest of each application localized to US customs.
Additionally, it is reasonable to make this setting available in Firefox as it is an application platform. Urging or expecting each application developer to handle this would be a never ending struggle, especially since many rely of on the functionality of the platform. For example, the newly released Frigate application version 0.17 includes removed functionality to configure how strings are formatted. Even in this application, there is the possibility to change the date format, but not the date format.
Considering all of this, I suggest that Firefox gets the ability to configure the date and time formatting.