What does the prompt to continue look like? Perhaps you could capture it and attach a screen shot to a reply (remove any personal information). This article has tips on that: How do I create a screenshot of my problem?

To see whether the dialog is generated by one of your add-ons, could you test Amazon in Firefox's Safe Mode? That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Use Troubleshoot Mode in Firefox.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

I have done what you said. Hope you can see the image. Thank you.

Based on what I found in a web search, this is designed to happen if you are shopping on the site of an Amazon affiliate instead of directly on Amazon.

If you are shopping on Amazon itself, two thoughts:

(1) You may have an obsolete cookie set. To clear your Amazon cookies (note: this will log you out), you can do this: While viewing a page on the site, try either:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

In the dialog that opens, you can remove the site's cookies individually.

Then try reloading the page and adding something to your cart. Any difference?

(2) You might have an add-on that is injecting itself into your shopping to try to get a percentage of your purchases.

Please try Safe Mode as noted above and see whether that changes anything.

Thank you for your help. Removing the cookies resolved the problem!