You may notice that when you go to take a credit card payment, the card number field has already been filled out. This is caused by Firefox thinking a previously swiped credit card was actually a password and it was accidentally saved.
You can resolve this issue in a few quick steps.
1) Click the Menu ☰ button in the top right side of your browser.
2) From this menu select "Preferences" if you are on a Mac, or "Options" if you are on a PC.
3) On the preferences page, select Security along the left side.
4) Click the Saved Passwords button. In this list you should see one that says https://hc.mercurypay.net or https://hc.mercurydev.net
5) Select that line, and click Remove to delete the saved credit card. Now when you go to take a credit card, you will see the card number field is blank and ready to go!
Important: It will ask you to save the password again the next time you swipe a credit card. When this window comes up, click the arrow next to Remember Password to select Never Remember Password for This Site.