Whenever I copy or move large files between disk partitions my firefox becomes unresponsive at random times during the process. Is that normal for Ubuntu?
This is probably because the CFQ I/O scheduler isn't being particularly fair for you. This probably happens because of a lot of dirty pages that need writing back to the disk and firefox is issuing file system syncs which don't really help. You may get some mileage in either tweaking the virtual memory system as follows:
echo 10 | sudo tee /proc/sys/vm/dirty_ratio
or changing the I/O scheduler to deadline:
echo deadline | sudo /sys/block/sda/queue/scheduler
where sda is the device name of the drive.