I'd imagine it's a large codebase and they're trying to maximize the number of people that can access it while minimizing the number of supported browsers. There are very few platforms where firefox is available and Chrom(e|ium) is not.
FWIW, it actually does work on Firefox Quantum; it's just not guaranteed to work.
Why no FF? What could possibly cause them not to support it? Any specialists out there?
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