Supported likely means that they test with and design for. It also means that if a customer is using that browser and there's a problem (e.g., seeing their bill online) they will fix the problem if it's just in that browser.
Firefox is 3.3% of users [1]. When it comes to testing features with (including QA validation) and working around browser specific bugs, what % of usage should drive that?
If we work to usage, we allow the browser market to be swallowed by one proprietary player.
That manufacturer can then call ALL the shots.
Engineers and development have a duty to work according to open standards to prevent this from happening.
Hardly thinking about their supposed duties.
So long as they don't active sniff and reject non-approved browsers.
I wish we didn't only measure (and pay for) five nines+ uptime, but also support. Remember how annoyingly often just 99.9% uptime means you're out of business?