Usually that's because of third party cookies the government websites love to use for authentication. FF and Safari by default blocks them but both can be disabled temporarily to use those websites. Chrome is more lax on them since ad networks love cross origin cookies as well.

I have no issues with mygov in firefox (on linux of all platforms). I don't even whitelist ublock origin on that domain. Check your other extensions.

I live in Australia and have been using Firefox for all my myGov needs without even thinking about it

Which service do you have issues with?

Just to test I just logged into myGov then through Firefox and it worked fine.

Wow. Force-Supporting the same company they're battling daily, on multiple issues.

MyGov works fine on Firefox but I do use incognito window with no extensions for them. I’d say it’s probably one of your extensions.