If a user brought you Lynx I'm pretty sure you wouldn't jump to support them. Whether Firefox deserves support by default is a question with a different answer at every company.
Twisting this into an absurd discussion does not give you the moral high ground, but fine, I'll bite.
If a browser is standards compliant it should be supported. Note that standards are there for a reason, they create a level playing field. So all you really need to do is to ensure that your implementation is conform the standard, you then still have to test to ensure that your assumptions hold in light of practical applications of that standard on the rendering side. But there really is absolutely no excuse why barring non-standard extensions your website would not work with all browsers with more than 1% market share.
If a browser is standards compliant it should be supported. Note that standards are there for a reason, they create a level playing field. So all you really need to do is to ensure that your implementation is conform the standard, you then still have to test to ensure that your assumptions hold in light of practical applications of that standard on the rendering side. But there really is absolutely no excuse why barring non-standard extensions your website would not work with all browsers with more than 1% market share.
https://en.wikipedia.org/wiki/Usage_share_of_web_browsers
Would be a good place to start for your compatibility testing.