Makes me wish for an extension that would temporarily - for a few days or weeks - cache everything you see under a given URL[0], and 1) tell you if the content changed since your last visit, 2) allow you to browse history of versions, and, importantly, 3) diff entries in that history.

Plenty of times I had a vague hunch the page said something slightly or perhaps entirely different just minutes or hours earlier, but have no easy way of checking it. Even if I have it opened in another tab, most likely the browser already decided to unload the content and reloads the page when I try to view it.

Bonus points if it could sync across devices - e.g. between phone and desktop using Firefox sync. Even recently, I had a situation where I was convinced someone edited their HN comment to say something 180° opposite to what it said earlier, in the 30 minutes between me reading it on a phone, and then reading it on a desktop later. Of course, the phone tab unloaded itself long ago, so I have no way of checking if it indeed happened, or was it just my brain misreading it the first time...

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[0] - +/- ignoring or employing some workarounds for SPAs that don't update URLs when the page content changes, or don't even use URLs to locate content in the first place.