I have the same problem, just installed the LastPass extension for Firefox and now whenever I use that browser to edit eBay items I get script appended to the description.
I found another post describing the same problem which suggests deleting your credentials in LastPass for the site and re adding them, this did not work for me.
"Last Past-Firefox. Printing scripts into ebay sell form. by Korrekte160 Fri Jul 29, 2016 2:23 pm"
If you didn't notice it was happening and fixed it you could be in breach of eBay policy for using scripts.