Portability / Compatibility: Does it work correctly on all modern browsers (excluding old versions of Internet Explorer)?
In Safari, there is a cute tingly-shiny (don't know how to call it) effect as the fully spread pieces fade out. In Chrome, either there is no such effect, or it's so faint it's not visible. (I'm on a Mac.) (Sorry, I don't know enough about animations / canvas to have clue as to why...)
Modelling: Is this a good way to simulate fireworks? Is there anything I could do to enhance the realism?
In the small view the snippet, it looks great. If you switch to full screen and look at it long enough, some minor warts start to show:
At end of the explosion, the pieces are all moving together, as if stuck on an invisible paper and floating down
The pieces seem to be moving downward at constant speed instead of accelerating, as if there's no gravity, or gravity is very week. It doesn't really look like they are falling
I suppose there is a time limit: after some time the fireworks just stop, though the airplane keeps flying through. And the fireworks don't stop gracefully, but in the middle of explosions, just frozen in the air.
Minor technical things
Semicolons are missing at the end of the definitions of Animation.prototype.start and Animation.prototype.stop. Which is valid, but not consistent with the rest of the code.
The greek variable names are cool...
var τ = 2 * Math.PI;
... it's just that, I don't know how to type them :-/. It would be simpler to just use English words.
The bottom line ...
Oh nevermind any of that! Big THANK YOU ALL for the amazing site and community! The announcement was unexpected and witty, very befitting, and pure awesomeness!