1: https://hacks.mozilla.org/2020/04/twitter-direct-message-cac...
2: If you have Firefox, browse about:compat to see Firefox's workarounds to websites which relies on Chrome-specific behavior.
3: Firefox implementing a wholesale change that maps "-webkit"-prefixed CSS properties (usually implemented by Chrome even, so Google should used "-blink" (the name of their HTML/CSS engine) since "-webkit" is for Safari's changes) to the standard and/or "-moz" equivalents for websites who are using it.
I can kind of see it both ways. I appreciate that Chromium browsers may have little extras you can tap into as it'll make for neat features (Project Stream and I imagine later Stadia is because of that sort of thing), but it does break browser compatibility so it runs the risk of replicating that "works in IE" experience from way back when.
The IE I don't specifically miss though was needing to support older releases of it at work, personally. But I could see disliking the "what do you mean it doesn't work in Safari/Firefox/etc.?" discussions that may happen.
IIRC all of them start as flags, and there are tons of them available, most disabled by default. Most move up to stable and enabled by default after some time and testing and feedback though.
Most commonly we'd see just weird things. Every other browser could handle "render this circle lightly colored, put some text inside it." But random versions of Safari would draw "solid black circle" with illegible text. Update the OS (the only way to update Safari) and the bug goes away. So we wrote work around after work around to make the same code that ran everywhere else run on Safari. And before you ask, all of our devs were on macs, but they weren't running random versions of the os where the bug was present. Or worse, the simulator in xcode wouldn't show the error but it would on a device. At one point over 90% of browser-specific bugs (and almost all customer-reported ones) were due to Safari or Safari mobile, despite making up less than 20% of our traffic.
Plus Apple keeps trying to stop people from running test infrastructure of every permutation of os and hardware. It's a nightmare and we just eventually put up a "safari may not work" warning on all our help docs.
For our team Safari was worse than ie6, because at least there everyone shared workarounds and knew how to handle the MS foolishness.
I should have worded it in a better way, and when someone said "X is the new IE" I should view the context of it. "Chrome is the new IE" is a very valid opinion, but not in all contexts, and so is "Safari is the new IE".