Let's say you want to run firefox using one CPU. You'd run something like this:
taskset 0x00000001 firefox
For allowing firefox to use 2 CPUs replace 0x00000001 with 0x00000003
For allowing firefox to use all CPUs replace 0x00000001 with 0xFFFFFFFF
taskset 0x00000001 firefox
For allowing firefox to use 2 CPUs replace 0x00000001 with 0x00000003
For allowing firefox to use all CPUs replace 0x00000001 with 0xFFFFFFFF