This is pretty darn cool. i've not yet seen any mention of one particular quirk so will point it out: on Firefox it flashes between "normal" and "upgraded" mode

This is pretty darn cool. i've not yet seen any mention of one particular quirk so will point it out: on Firefox it flashes between "normal" and "upgraded" mode pretty starkly (A) when switching pages and (B) when tapping "add comment" (which is apparently done via page reload rather than the HN API). i've no clue whether an extension/app has any influence over the timing of that so cannot suggest how it might be resolved, but will say that it's particularly jarring.