Possible I suppose. You can restrict firefox memory usage in the config. Perhaps their dynamic allocation was getting confused by what was available on the 64 bit machine? Still. Why would any 32 bit app even try to allocate more than it actually could handle. I dunno. I'm still inclined to think missing libs (or out of date libs) - but hard to say without a bit more detail on the crash. Did anything show up in .xsession-errors / stderr ? Were you able to launch it in a clean profile? Were the crashes visible in about:crashes for the profile when launched in 64 bit? I suppose it doesn't matter too much at this point...