The memory reporting infrastructure currently only properly handles one child process. If multiple child processes are present, they will overwrite each others' memory reports in the parent process, such that only one child process will be shown in about:memory. The cross-process memory reporting design is awful for a bunch of reasons, and needs to be completely overhauled. Fortunately, we don't need to do that for this bug; it can be band-aided fairly easily once bug 910517 lands.

> If multiple child processes are present, they will overwrite > each others' memory reports in the parent process, such that only one child > process will be shown in about:memory. Turns out this is wrong! I thought there was a single ContentParent in the parent process, but there is actually one ContentParent per child process. As a result, about:memory works fine with multiple child processes.