It doesn’t even work on Chromium, in the same version as Chrome. There is literally no excuse for that, it’s literally the same browser engine.
On top of that, even back in the "best viewed on netscape navigator 4.0" era there was a solution for this: Show a message that it was only tested with browser X, and that your client was too cheap to pay for anything else, but at least allow the user to bypass that.
As said, the excuses convinced me even more to never do business with you.
Moreover, you failed to acknowledge the points they made in response!
The loop of "everyone only uses chrome" → "I only need to support chrome with my website" → "nothing works on firefox, I'll switch ti Chrome" is harmful to the entire internet industry, and the startuo economy.
It is harmful to all of us, and hurts all of our future.
I see their situation, and their limits, but this is a very egocentric opinion that completely ignores the effects on the rest of society, which is an issue that's just all too common (see also monopolistic effects, or environmental effects, etc).
Given the target audience (graphic designers, the vast majority of whom are on Mac and will use Safari or Chrome, as I validated from stats on other sites I've built), the decision was made to focus work on the type tester on those browsers.
The rest of the site should still be accessible with any browser, but we decided it was better to not show the type tester at all than have a broken experience (which was the situation with Firefox).
Anyway, hopefully that justifies it somewhat - I think it was the right decision all things factored in. There's a lot of CSS trickery/hackery to make the type tester work (native support for advanced typography stuff is poor) so making it x-browser wasn't easy. The messaging to users of other browsers could probably be improved though :)