Is this arguably anticompetitive and illegal?

Nobody can even state that it’s actually happening “for competitive browsers” as even Chrome users are reporting an unexplained lag/slowdown. At this point, it’s just wild speculation and bandwagoning.

You absolutely can tell what's happening by reading the source code. They are using a listener and a delay for when ontimeupdate promise is not met, which timeouts the entire connection for 5 full seconds.

function smb() { var a, b, c, d, e, h, l; return t(function(m) { a = new aj; b = document.createElement("ytd-player"); try { document.body.prepend(b) } catch (p) { return m.return(4) } c = function() { b.parentElement && b.parentElement.removeChild(b) }; 0 < b.getElementsByTagName("div").length ? d = b.getElementsByTagName("div")[0] : (d = document.createElement("div"), b.appendChild(d)); e = document.createElement("div"); d.appendChild(e); h = document.createElement("video"); l = new Blob([new Uint8Array([26, 69, 223, 163, 159, 66, 134, 129, 1, 66, 247, 129, 1, 66, 242, 129, 4, 66, 243, 129, 8, 66, 130, 132, 119, 101, 98, 109, 66, 135, 129, 4, 66, 133, 129, 2, 24, 83, 128, 103, 1, 255, 255, 255, 255, 255, 255, 255, 21, 73, 169, 102, 153, 42, 215, 177, 131, 15, 66, 64, 77, 128, 134, 67, 104, 114, 111, 109, 101, 87, 65, 134, 67, 104, 114, 111, 109, 101, 22, 84, 174, 107, 169, 174, 167, 215, 129, 1, 115, 197, 135, 207, 96, 156, 234, 24, 157, 175, 131, 129, 1, 85, 238, 129, 1, 134, 133, 86, 95, 86, 80, 56, 224, 138, 176, 129, 1, 186, 129, 1, 83, 192, 129, 1, 31, 67, 182, 117, 1, 255, 255, 255, 255, 255, 255, 255, 231, 129, 0, 160, 204, 161, 162, 129, 0, 0, 0, 16, 2, 0, 157, 1, 42, 1, 0, 1, 0, 11, 199, 8, 133, 133, 136, 153, 132, 136, 63, 130, 0, 12, 13, 96, 0, 254, 229, 106, 0, 117, 161, 165, 166, 163, 238, 129, 1, 165, 158, 16, 2, 0, 157, 1, 42, 1, 0, 1, 0, 11, 199, 8, 133, 133, 136, 153, 132, 136, 63, 130, 0, 12, 13, 96, 0, 254, 232, 120, 0, 160, 187, 161, 152, 129, 3, 233, 0, 177, 1, 0, 47, 17, 252, 0, 24, 0, 48, 63, 244, 12, 0, 0, 0, 254, 229, 106, 0, 117, 161, 155, 166, 153, 238, 129, 1, 165, 148, 177, 1, 0, 47, 17, 252, 0, 24, 0, 48, 63, 244, 12, 0, 0, 0, 254, 232, 120, 0, 251, 129, 0, 160, 188, 161, 152, 129, 7, 208, 0, 177, 1, 0, 47, 17, 252, 0, 24, 0, 48, 63, 244, 12, 0, 0, 0, 254, 229, 106, 0, 117, 161, 155, 166, 153, 238, 129, 1, 165, 148, 177, 1, 0, 47, 17, 252, 0, 24, 0, 48, 63, 244, 12, 0, 0, 0, 254, 232, 120, 0, 251, 130, 3, 233 ])], { type: "video/webm" }); h.src = lc(Mia(l)); h.ontimeupdate = function() { c(); a.resolve(0) }; e.appendChild(h); h.classList.add("html5-main-video"); setTimeout(function() { e.classList.add("ad-interrupting") }, 200); setTimeout(function() { c(); a.resolve(1) }, 5E3); # <------------------------ 5E3 = 5s return m.return(a.promise) }) }

I’m sorry but I don’t see how that check is browser-specific. Is that part happening on the browser side?

They don’t need to put incriminating “if Firefox” statements in their code – the initial page request would have included the user agent and it would be trivial to serve different JavaScript based on what it said.

Does it though? You cant just speculate that something could happen and insinuate that it is without evidence.