I was curious as well so I looked at the git tree. I'm not familiar with Firefox code, but I'm assuming I found the list:

pref("extensions.webextensions.restrictedDomains", 
"accounts-static.cdn.mozilla.net,accounts.firefox.com,
addons.cdn.mozilla.net,addons.mozilla.org,
api.accounts.firefox.com,content.cdn.mozilla.net,
discovery.addons.mozilla.org,install.mozilla.org,
oauth.accounts.firefox.com,profile.accounts.firefox.com,
support.mozilla.org,sync.services.mozilla.com");

From here

So it looks like it's mostly to do with the account system of Firefox. I'm not sure why their websites would need special protection, but whatever. It's not malicious, for now