Well the keys Ctrl+Alt+Delete are all pressed at the same time, this opens a screen that allows you to start the task manager.

 This will open to the Applications tab of task manager, you will notice that there is another tab called Processes, here you would look for firefox.exe click on it and open it with a right click, here you will see end process.

Thank you - but what do I then do at the 'end process'stage?

Thanks.......Sorry but I am only a BIT computer literate!!!!!

Well you would click on the program you want to stop, in this case you said you wanted to stop Firefox so that would be in the list as firefox.exe

 Once you see that it is highlighted click End Process, at this point you may get a message that the process is busy. In your case you would tell it to continue.
 Of course it would better to find out way this is necessary to do, it is not a normal way to close a program nor should it be used unless there is no choice.
  But in saying that it is better than shutting off the power switch, killing the power causes many open processes to hang and fail on start up. Of course most of the time Windows will ignore these and continue on. This eventually(if done often enough) will lead to fragmentation and slow down drive access.