I would suggest to start, unplugging your power supply, then removing all DIMMs, and reseating only one back in (in the recommended slot by your motherboard manual). Try then to start.
I don't understand what you mean by "plugged in firefox". Do you mean you installed it? Ran it? Regardless, if you downloaded and installed the 64-bit version of Firefox then I'd suggest uninstalling it and re-installing with the 32-bit version instead.
Davemon50