It's not so much having trouble as just inconsistency of implementation across platforms. For example, I usually do this kind of work on Windows. I certainly could spin up a quick Ubuntu VM and run Firefox in that, but various aspects of the page rendering might change as a result. Given that far more Firefox-using visitors on most real sites will be running Windows than any other platform, testing with real Firefox on Windows is less error-prone. Ditto for Chrome, etc.