First of all, one of the two screenshots
must be doctored, unless Firefox 3.6.3's PNG decoder does not respond deterministically to a particular PNG file. Moreover, for your theory to be correct (it consistently fails on my machine but actually works for everyone else) it would have to consult not even an RNG but some stable feature of the machine it's running on, like its MAC address, to decide whether that particular image should be decoded or treated as "having errors".
Tell me, now, why Mozilla would design Firefox to check the MAC address (or whatever other equally-irrelevant stable feature of the machine it's running on) to decide whether to decode a PNG normally or to falsely state that the PNG file contains errors? This seems to amount to a conspiracy theory with no apparent motive and a particularly implausible villain, whereas someone being embarrassed by their encoder not being compatible with a browser with significant market share, wanting to pretend it isn't so, and a couple other people jumping misguidedly to their defense is far more plausible.
As for the notion that it's running out of virtual memory, or anything similar:
1. I viewed another 8000x8000 png with FF recently without issue, on the same hardware and with the same copy of FF.
2. The hardware in question has 3GB RAM.
3. 22 megs is a joke compared to 3GB.
4. The error is not "out of memory", it is "the file contains errors". It clearly indicates that the FILE ITSELF is wrong, not something else in the decoder's environment, such as the available memory.
5. There is very little data downloaded before it gives up and says the file contains errors. To judge by the screenshot, it reads the PNG header, determines the width and height and the scale-factor to use initially when displaying it, then sees something in the PNG header that it doesn't like, closes the stream, and reports an error. This points to a header that is, according to the decoder in FF, malformed -- probably because it uses some newer PNG feature that FF's decoder doesn't support. This fits the incompatibility theory much better than it fits any theory that makes the problem somehow local to my machine. Out of memory, in particular, would manifest with it spending a while chugging downloading data and then horking it all up at some point with an error message saying "out of memory" or even an outright crash.
Once again, I invite skeptical readers to click the link in FF 3.6.3 themselves and see what happens. I also invite them to examine the rightmost taskbar button in the screenshot purporting to show FF 3.6.3 successfully decoding the png at issue. It does seem to be true that the user who posted that screenshot does not, specifically, use Adobe Photoshop ...