After some experimentation, it turns out my problem was that I expected - ridiculous, I know - that it would copy image data in some form that's comprehensible to Gimp, Firefox and Thunderbird. So I now rather suspect that Photos is using some new API, that Gimp/Firefox/Thunderbird aren't, and that this new API is, for no particular reason that I can see, not remotely backwards-compatible.
Feels like this forms a data point for the thread - since meanwhile, on Windows, I have a couple of programs in my bin folder that I use fairly regularly that I compiled in 2006.
Feels like this forms a data point for the thread - since meanwhile, on Windows, I have a couple of programs in my bin folder that I use fairly regularly that I compiled in 2006.