Next year, I plan to fly from Europe to China around new year's eve. That's why I was asking myself whether it is possible to see fireworks on the ground from an passenger aircraft traveling at cruising altitude. If it's not possible at cruising altitude, maybe it's possible during take off or landing? Any estimates on the maximum altitude?
Some years ago I was a part of an operating cabin crew in a flight bound to Dubai, it was the time for one of Dubai's festivals.
Anyway, I was positioned at the back and at the time we already started the descend, the crew chief called me and asked me to take a walk around and ask people to sit because many were standing! the plane was a narrow-bodied McDonnell Douglas.
I went to the aisle to check, and they were standing because the fireworks were actually visible at that altitude (can't remember exactly, but it wasn't the cruising altitude nor it was very low). They were only visible from one side and people in the other side were standing to be able to see it from the other side's windows! I spent a few moments enjoying the view until I remembered that I was a crew member.
Bottom line, yes they are visible from high altitudes, and if the weather was clear I guess they would be visible from the cruising altitudes as well.
I haven't personally seen fireworks from a plane, but there are quite a lot of videos available which do show fireworks, even from high altitudes.
This is a video taken at 36000 feet, which should be around cruising altitude. Bright blinking lights can be seen, but they do not really look like fireworks, more like camera flashes.
Here is a video showing fireworks at a lower altitude, where you can actually make out that it is fireworks.
Here is a video showing fireworks during takeoff, which is interesting, because you can see how the fireworks look at different altitudes.
And finally a video which seems to be taken at a lower altitude, which does get quite impressive around minute 2:30.
So the answer is yes, you can see fireworks, but I think it's mostly not that impressive, especially at cruising altitude. I would imagine that it will be even less impressive if you are not directly above a city, but just passing near it.
Austrian Airlines does an annual Silvesterflug (New Year's Flight). This year it took off at around 22:50 and landed 00:30, flying rounds above central Vienna.
While this is possibly the best possible conditions giving that it's a flight specifically for looking at fireworks, I'm not sure how much you can actually see from above. The demand was sufficient to justify two flights this year. I believe it's done in either their Fokker 100 or Airbus A320 aircraft, so a pretty average window size.
I flew from Manchester to Southampton (both in the UK) on 5th November a few years ago. There were lots of fireworks going off for the whole trip (both in back gardens and formal displays) but from the air they were most unimpressive.
Interesting to see, but I wouldn't go out of my way to see it again.
A few years ago, flying on a plane to Israel during the night, I idly calculated the minimum brightness of a light visible from cruising altitude.
The calculations are easy: brightness is proportional to inverse square of the distance, and 5 (stellar) magnitudes is 100 times the brightness, so something that is magnitude -11 (a bit fainter than a full moon) at 10 meters away will, in principle, be as bright as a fourth-magnitude (dim but very visible) star from cruising altitude.
I hadn't calculated further to see exactly how bright this is, but "as bright as a full moon, when viewed at 10 meters" is probably somewhere around candle level (certainly much less than a properly-pointed flashlight).
Naturally, your typical firework will be a good deal brighter; probably bright enough to be brighter (as visible from cruising altitude) than any actual (night-time) star.
However, it would also be far away, and thus rather tiny; the (typical) limit of human vision is about 1 angular minute (1/60 of a degree), which corresponds to about 3 meters from cruising altitude.
Thus, a firework that blooms to 10 meters in width will be 3 angular minutes across - basically a fat bright spot; you might not be able to tell it was a firework. I do not, however, recall how large a typical firework is; it might be larger.
Also, most fireworks are typically in cities, so they might be drowned out by the regular city lights (remember, we're talking about 3 meter resolution).
I can tell from personal experience that car headlights are very visible from cruising altitude, as little bright (and slowly moving) dots along the roads; of course fireworks are much brighter. I've never been on a plane at a time when fireworks would make sense, so I can't say I've ever seen any that way (if I did - which is well possible given how many random fireworks there are for stuff like somebody's birthday - I would not have recognized them as such).
During takeoff and landing, of course, this all becomes magnified; night-time landing at Tel Aviv airport is fantastic - coming in from the sea, one can easily see all the lights of the city. No fireworks that way either, but I can imagine that it would've been very pretty.
All of the above relates to night-time, incidentally. You would likely have a lot less chance during the day.
Took a night flight from London Heathrow to San Francisco about 15 years ago on Bonfire Night and saw flashes of fireworks over Britain. Quite distant even though we were probably not at full cruising height. They were pleasant to see but not much definition. However I do remember there were none as we flew over Ireland, and it was a few minutes until I realised it may be because Guy Fawkes was a Catholic. I also heard that fireworks may be outlawed in Ireland.