With the default amount of memory, Dehydra frequently runs out of memory when compiling Firefox source code. With this increase, I was able to compile the JS directory and a few others without problem.
Created attachment 538389 [details] [diff] [review] increase mem allocated to JS runtime by almost 20 times Something a little less might work, too, but I think I was having problems even at quadruple the original memory.
js/src/jsopcodes.cpp was the main offender, though at the default setting there was flakiness on other files. I went back and tried slightly more fine-grained settings, and it was running out of memory for me at 0x256L, but 0x384L was okay.
512 also works for me, FYI, which is what jcranmer pushed http://hg.mozilla.org/rewriting-and-analysis/dehydra/rev/ef77d62c0dbd