If you're developing software for one client who only uses a specific browser, I can see this being okay, but several times I have chosen not to buy things from websites that were broken in Firefox. I don't bother to check whether they'd work in Chromium, I just buy it elsewhere.
The number of people who act like me probably isn't large in absolute terms, but how many customers have been lost because of a broken website that you didn't even know about because they just left without a trace?
This might not apply to you, but it's some food for thought whenever Web developers decide to be sloppy and not check compatibility for a browser that still has significant market share.