You can type
firefox &
to tell firefox to run in the background, then you can keep using your terminal session...
Edit: And @ByteCommander has explained this much more thoroughly than me.
But sometimes Firefox (or another process that has been told to run in the background) will print some warnings and so on to the terminal, even if the process is running in the background and has been disown
ed. If you don't want that you can tell it to be quiet by redirecting stdout and stderr
firefox >/dev/null 2>&1 &
and disown
if you like. This will give you the PID of the child process and send any output from it straight to data Nirvana (@Videonauth's phrase), so you can carry on in the blissful certainty that you won't be interrupted. Note that you have to do this when starting a process; it can't be added later.