This only effects computers where the till has been set up to not pop up a dialogue box in when printing from Mozilla Firefox.
Close all sessions of Firefox. Open up a new session. in the address bar enter in about:config and press enter.
You may get a message regarding Here be Dragons.
Select "I'll be careful, I promise" you will be taken to the Mozilla Firefox hidden config page.
At the top of the page in the Search box, type in print.always_print_silent
You should see a result which is set to True. Double click this and set to false.
Then close this session down and open up a new session. Log into RPii as normal. This does now mean that you will get a print dialogue page when you do a receipt - but the good things is you can click cancel if the customer doesn't want a receipt.