Firefox has a feature to automatically correct URLs that are mistyped. For example, if you type wikipedia.prg in the address bar, Firefox will automatically correct it to wikipedia.org.

This also world for other root domains like .com and .net, and also for URI schemes like https.

You can find more details about this feature in the source code:

https://phabricator.services.mozilla.com/source/mozilla-central/browse/default/docshell/base/URIFixup.jsm;cad2659a1a1678a5be2c8874752586441b0a8ae0?as=source