Hi, try turning off 'Enhanced Tracking Protection' for that site (and if necessary, each stage of the payment process) by clicking the shield icon at the left-hand end of the address bar while the page is loaded - Enhanced Tracking Protection in Firefox for desktop.

Hold down the Shift key when you start Firefox to start in Safe Mode. More - Diagnose Firefox issues using Troubleshoot Mode.

If it works in Safe Mode, disable any theme and your extensions - 3-bar menu > Add-ons > Themes/Extensions, then restart Firefox; now if it's still OK, re-enable them one by one until you find the culprit. You may need to restart Firefox after enabling some extensions.

If your question is resolved by this or another answer, please help other users by marking the best reply as Solved. Thank you!

Thank you Scribe! It took a while but I have discovered that it was Privacy Badger. I am now wondering whether it has started doing this since they changed their default way of identifying trackers, as described here: https://www.eff.org/deeplinks/2020/10/privacy-badger-changing-protect-you-better I have no idea whether their 'local learning' (previous default) was actually a security risk. if not, I'd prefer to return to that, rather than keep disabling PB for individual transactions or sites.