My best guess is the dates on which the feature was added, which can also be seen on CanIUse. Firefox added OPFS support in March this year, and much of the userbase (AFAIK e.g. Firefox ESR) is still lacking the feature – in any case it's a very recent change on Firefox. However, webkit/Safari has had OPFS for over two years by now. I was personally unaware of the support having been added to Firefox as well, last time I checked the discussion they told they weren't going to implement the API.

By no means is this an acceptable excuse in my opinion, this kind of check should always be done by checking the existence of the feature, not the UA string. Though it might be that the check is still performed in the correct way as Safari users stuck on older version are also encountering the issue. But if they're fine with using OPFS, where you need to export the files separately to access them outside the browser context (as the storage is private), there's no reason to complain about recent Firefox versions that support this feature.

But, the same point still stands, kind of. The main underlying problem is Google forcing new standards through Chromium, without waiting for industry consensus and a proper standard. Then, as 80% of the userbase already has the feature everyone else is forced to get on board. I still don't really see Adobe as the main culprit here, despite the apparent incompetence in writing compatibility checks, but Google with their monopolistic practices with the Chromium project. Adobe isn't innocent and has done the industry a lot of harm in the form of being one of the original pushers of subscription software, but I don't think this instance should be attributed to malice rather than incompetence.

Edit: So, a bit of additional advice for someone trying to get this to work: in case the UA spoofing doesn't help, check the Firefox version in use – it has to be 111 or newer, as 111 was the release where File System API support was added. Firefox ESR probably doesn't have it available. Also check that FS API / OPFS doesn't need to be enabled through some flag or configuration parameter, and that it's not blocked by some plugin.