Just tested my add ons to firefox one by one and found Ublock is the one causing it to not work. I had to go into settings and disable it before ebay would work. If you click on the icon in the upper right and just turn it off for that site it still would not work. You have to go in and disable the addon
I came to the same conclusion yesterday. It's unfortunate – I can't see Ebay changing anything to accommodate uBlock.
Meanwhile, F5 is your friend, but an annoyance.
Lot's of other annoying 'features' in Ebay these days, like Lastpass auto-filling the search box and Paypal no longer being available outside a modal window (so Lastpass won't work to log in to Paypal from within Ebay – copy & paste is back).
In reference to my original post, the thing is that this copy of FF has NEVER had ANY extension installed. So uBlock CAN NOT be the the problem as it's simply not there.
An uninstall/reinstall of FF fixed the issue anyway, so I I don't agree it's an eBay problem.