Open File > Page Setup
Make all the headers and footers blank
Set the margins to 0 (zero)
In the address bar of Firefox, type about:config
Search for print.always_print_silent and double click it
Change it from false to true
This lets you skip the Print pop up box that comes up, as well as skipping the step where you have to click OK, automatically printing the right sized slip.
If print.always_print_silent does not come up
Right click on a blank area of the preference window
Select new > Boolean
Enter "print.always_print_silent" as the name (without quotes)
Click OK
Select true for the value
You may also want to check what is listed for print.print_printer
You may have to choose Generic/Text Only (or whatever your receipt printer might be named)