Maybe because browsers are now released every few weeks and version is incremented and so it is now impossible to identify a browser main version (like let's say IE11 including none or tons of patch but still named IE11).
And I doubt Oracle will update all their certification matrix every single time Firefox or Chrome decide to release a new version.
So it makes sense that the matrix says 17+, they just didn't update it anymore ....
Oracle new reply. Finally, it's really 17+, so 38 is supported.
Well, at least they have an opinion, not really fixed and changing over time but an opinion
At some point they will have to limit the 17+, mainly if there is a major change in FF coming, but good for you it's working with FF 38 (it's anyway time to start thinking at what your upgrade plans are).