Credit card industry PCI-DSS security requirements required credit card processors (PayPal) to implement at least TLS 1.1 security protocols in browsers for transactions by June 30th.
Older browsers Firefox versions less thv v24, etc) only support TLS 1.0 and can no longer be used with the PayPal website or to checkout with PayPal on sites like eBay.
Firefox 52 does TLS 1.2 ( more than good to meet the requirement), and is the last version to run under Windows XP. OthEr browsers (IE, Opera, Chrome, Safari) also have versions that will comply and can be run on older OSs.
(If running Win 95, 98, or ME, or a REALLY old Mac OS, you are probably out of luck)
What OS (Win XP, Win7, MacOS 9, etc), and what browser/browser version (Firefox 21, Safari 8, IE 9, etc) are you using?
See here for more detail:
https://community.ebay.com/t5/Selling/Why-is-it-eBay-telling-me-have-an-outdated-browser-when-my/m-p...https://community.ebay.com/t5/Technical-Issues/Update-browser/m-p/28671598