Sadly Firefox has been out of our browser matrix for several years now, it is only taken into consideration by FE teams when the customers explicitly ask for it being supported.
I also use because I care, but at 3% hardly any business does any longer.
I think I understand where he's at. If your web site has compatibility issues with smaller browsers like Firefox at 3%, Opera at 2% etc. then you could be losing out on 5% of your sales. If you were to approach any CEO and ask if they'd be interested in an initiative to increase sales by 5%, they would most likely express an interest.
I mean, I don't object in principle, I in general consider this to be "doing a good job" that we all strive for, but in this particular case it was a "line of business" app with like 500 users so I genuinely hadn't even considered it. We'll see if it comes up later!
IE6 was already announced dead by Microsoft when it reached so tiny market share, you are forgetting IE got several versions up to 11, before being replaced by Edge, followed by Edge Chrome, later rebranded as Edge.
I also use because I care, but at 3% hardly any business does any longer.