With firefox's market share of about 10%, the "we don't support firefox" message is basically announcing "our website is useless to 1 in 10 people". Very frustrating.
10%? All the statistics
I've seen recently put firefox bellow 5% and often well bellow like 3-4%. Given that is less common than colorblindness, colorblindness is not all that well handled on the web, and not handling a disability is an actual legal liability..
But then you wouldn't post your product in the "vision impaired community forums" for example. I'd be curious to see the actual stats, but HN surely has a high percentage of Firefox users compared with the general tech-naive population.
This isn't a "show HN" so most likely the author is not the poster. HN is also probably more color blind than average, so the poster should have checked multiple browsers and then the palettes before finding it interesting enough to post?
I get the idea that it would be nice if the web were different, but I think that is a personal responsibility to choose for yourself or try to address yourself by submitting a PR that makes the repo better.