That's not an installer, that's a device page.

It's the specific functionality needed here that Firefox lacks that makes the /e/ page show the warning, unlike the lineage page that does not have the problem in the first place.

The fun part is that I got to this installer page by clicking on "Check device compatibility" on the https://e.foundation/e-os/ page..

So I was actually expecting a device page, not a WebUSB program..