There's really no excuse. When I started web development I had at least 3 browsers open at any given time and writing code that worked in all major browsers could be really difficult at times. In more recent years, I would just develop with one, quickly check if everything looks alright in the other ones, do some super-minor adjustments, and call it a day. Sure, some minor stuff may not be supported by Firefox, but it's never deal-breaking stuff. Safari is the only browser that regularly requires more work, but it's nowhere close to the troubles that every single IE caused.