Firefox has the great feature of saving addresses so that they can be autofilled on forms. I am aware that the birthdate is actually not part of the address. But with the address autofill we have the infrastructure to save form data and there are so many forms where I have to enter my birthdate (especially in December with all the online advent calendars where you can win something 😉) which is often the only field I have to enter manually as the rest is autofilled with the saved address data. At least if the form uses a date field and not a simple text field, it should be easy to support this use case reliably, provided that the field is named accordingly. So it's less a question of technical feasibility, but much more a question of whether you _want_ to expand the feature by this field even if it's not part of the address. Or maybe we should just call it form autofill instead of address autofill? 😉