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So tired of Adobe. They're part of the problem.
(images4.imagebam.com)
A place to discuss the news and latest developments on the open-source browser Firefox
In my experience, companies typically use this message because they only officially support/test on the listed browsers. They block out any other browser to cut down on support emails. If you spoof your user agent so the site thinks you're using a supported browser, somewhere between 95%-100% of the functionality will usually work just fine.
That's what our devops team does. All of my company's internal web apps are tested against chrome only and that's all that's supported. It will usually work with Firefox so it's not a big deal unless you have an issue and they find out you're not using Chrome.
Companies should test to open web standards and tell users to file a ticket with their browser devs if something breaks because of nonstandard behavior.