I had the same problem till today. And the solution was to disable Dark Reader add-on for Modzilla (same for Chrome too). It's very nice add-on, saving the eyes for long work on computers, but seems it have conflict with Paypal verification system. I guess other add-ons may have the same problem, so you can disable them all before login to Paypal. Another solution for me was to login using my 2nd computer on which I don't use Dark reader. Hope this helps.
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