If the product you are building is not future proof it is your problem not Mozilla's.Nothing is future-proof if the people controlling the platforms move the goalposts. We have standards and value backward compatibility for a reason: it's because violating those standards and breaking that compatibility hurts. And it's going to become Mozilla's problem if they continue down this path, because Firefox will cease to be a viable browser choice for a significant proportion of their potential market.
Dont blame Mozilla for your poor technology choices. Applets will eventually stop working
I don't know why you're writing as if I personally broke these medical devices. I've never personally worked on any of those projects, I'm just familiar with them and citing them as examples of why this sort of change is damaging.
In any case, people really should get off the "Java applets is a dead technology" bandwagon. Viable replacements using HTML/CSS/JS are very recent developments, and people have been developing web-based user interfaces for all kinds of devices for decades. Of course they're not all going to throw out all that work and rewrite everything from scratch. There's nothing wrong with it, and contrary to your claim, there is no reason those applets must eventually stop working. They'll work just fine as long as browsers run Java applets, which they've been doing just fine for many years.
Obviously applets will stop working if browser makers deliberately drop support for them even though it's been available for a very long time. However, that's like saying obviously CSS3 is no use for anything because it's not all W3C standardised in stone yet so you're stupid if you use it today because it might all be different at some arbitrary point in the future that no-one can predict. You can shoot down any technology, no matter how modern and trendy, with such a generic argument, but it doesn't demonstrate anything particularly helpful to do so.
You broke the medical staff instruments by choosing or maintaining a dead technology thus putting patient lives in danger.
No, I didn't, but if the serious software I work on were medical in nature, you could literally bet your life that I wouldn't be letting either Java or Firefox anywhere near it and would be using an entirely different level of engineering practice to build it, as I do for certain projects in other fields where reliability is essential.
However, while we build the literally life-or-death systems that way, chances are the word processor, KVM, and, yes, web browser that organisations doing vital work use for day-to-day activities are not developed to the same standards. Breaking them still hurts, if only in efficiency (which can obviously still be harmful in a medical context). Unless you're claiming that all software that runs in any medical facility must be developed to the same standards as control software for high-risk, safety-critical systems, again, your argument is so generic that it doesn't really prove anything interesting.