What if there's a bug in Mozilla's implementation at some point and DoH servers have to return a slightly different response for certain versions of Firefox? How will they achieve that?
That seems less like a fix than a bandaid applied at the wrong level of the stack. It's perfectly normal for websites to serve slightly different pages to different browser versions, but I would submit that is not a desirable property for a system to have but rather a symptom of our inability to properly standardize & implement something as complex as a browser.