Looking at your last post, you have DuckDuckGo Privacy Essentials and uBlock Origin installed. You have the older version for uBlock Origin, 1.60.0 instead of 1.61.0. But your DuckDuckGo addon is up to date.

Try updating uBlock Origin to see if that helps as ad blocking addons can sometimes block captchas from working properly or even appearing at all. Update link: https://addons.mozilla.org/en-US/firefox/addon/ublock-origin/

If that doesn't work, I would disable both addons, restart the browser and try logging into Ebay again.

And if this still fails, I feel like this has to be the built-in Tracking Protection of Firefox blocking a script or cookie the captcha needs to work and allow to you to login.

So I would set the Tracking Protection to the Standard mode: https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop#w_adjust-your-global-enhanced-tracking-protection-settings

If that still doesn't get it done, try disabling Tracking Protection directly on the Ebay site: https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop#w_what-to-do-if-a-site-seems-broken

The thing is that I started Firefox in Safe Mode, which is supposed to disable all add-ons - the problem persisted. uBlock Origin is already up to date in my browser. That being said, I tried it this morning and it is working again. I did download the Beta version of Firefox, and it worked fine with eBay with all of the add-ons enabled.

So, I have no idea what was causing the problem, but everything is working again, so rock 'n' roll.