With great power comes great responsibility.
But in earnestness, it is not Apple's responsibility only till the point where it is not something of their doing in the first place that they are working to fix.
Things are not so black and white where we can make off-the-cuff statements like that.
Case in Point: I use Rapidweaver and a plugin was not working owing to notarisation issues. Two betas later, the plugin started working. This meant that Apple fixed something in their software that (apart from other things) was also causing the plugin to not work. Nothing the developer of either the plugin or Rapidweaver could do in that case.
It becomes Mozilla's responsibility when Apple makes a change that requires developers to make compatibility changes - this becomes clear to the developers early on and they are usually quick to confirm to their user base that a fix is in the works and will be available by the time an OS releases. So long as that is not the case, all "breakups" are Apple's doing
and the onus to fix that lies with them alone.