What is the content of this error message?

Can you attach a screenshot?

• http://en.wikipedia.org/wiki/Screenshot
• https://support.mozilla.org/kb/how-do-i-create-screenshot-my-problem

Use a compressed image type like PNG or JPG to save the screenshot.

Clear the cache and cookies only from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe Mode start window.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thank you for helping. But the Error message simply says, "Wrong Card Type" when I am sure that the card type is entered correctly.

Try to disable form fill to see if that helps.

• Tools > Options > Privacy > Firefox will: "Use custom settings for history": "Remember search and form history"
• https://support.mozilla.org/kb/Form+autocomplete

What is the site if you do not mind me asking?

Maybe the site has problems. Or is it this site and you are trying to donate for Firefox.

I typically clear my "History" fairly regularly. I can't find a "Tool" option on my Mozilla browser.

I click to donate from the Mozilla web browser option or from the email sent to me by Mozilla. I get the same Error Report saying that there is something I am not filling right. But this is a credit card I have used for other purchases on the web without problem. I notice that Paypal manages the process. Can it be that I have no credit card listed on Paypal?

This page? https://sendto.mozilla.org/page/contribute/

It accepts Visa and Mastercard credit cards and would expect a 16 digit card number and 3 digit security code.

If those are normal, maybe it's a Paypal issue. You do not need to register your credit card with Paypal in advance to make a one-time payment.