Clear your browser's cache and cookies: Go to Options > Privacy & Security > Cookies and Site Data, and click on "Clear Data". Select both "Cookies and Site Data" and "Cached Web Content", then click "Clear".

Try using a private browsing window: Open a private browsing window and try making the payment again. This will disable any addons or extensions that may be interfering with the payment process.

Unfortunately, none of the above worked for me. The problem seems to lie with the latest Firefox update.

You already tried with Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

What security software are you running? Do you have the same issue under a different windows account? If you don't have one, make it or use the admin account. If you're running Windows Pro version, try the same steps in windows sandbox (VM) and see if you have the same issue. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

I just made a purchase a few days ago with no issue. see screenshot

Yes, safe mode was used and VPN/firewall was turned off.

I can also make payments on some sites, but when there's a frame involved it's no go. I'm having to use Chrome to get these types of payments through.

Windows pro or home?

I'm using 10 home version..

Issue fixed, hard refresh after switching off vpn corrected. Thanks for the responses!