In which way could Mozilla be coerced in building a backdoor into its software by the Australian government, in a way not being seen by the public? I can't imagine that an Australian law would either lead to shutting down open source projects nor lead to make them (or parts of it) closed source. Would Signal do this? I don't think so, they'd loose all trust immediately.

The only problem I see is that open source projects might not accept contributions from Australian residents or citizens.

Presumably in a way similar to Mozilla implementing closed source DRM in Firefox?

I can switch that off (afaik). And the question remains: how can Mozilla compelled to do this?

Couldn't they just put another version in their websites, the Australian version, that includes the backdoor, while proposing the en_US and en_UK version along them that don't include the backdoor ?

That coupled with a disclaimer saying that only Australians should download this version because of the security implication and saying the other should download other version would make them comply while communication to the users that they should install the non Australian version, even in Australia

Please accept my apologies for the idiocy of our politicians. If it makes you feel better, they are generally too technologically incompetent to actually enforce anything like this.