E-mails are sent using Outlook. After your post, I tried sending a link to this page and got a message "Page not found". I had sent several links from different webpages today and they worked fine. However, after reading your post I decided to change the e-mail account from which they were sent (I have two different e-mails that I can use from Outlook). When I changed the "sending address" the pages and links worked fine, so that solves the problem. Although why I just started having this problem with e-bay, I have no idea. I have made no changes recently in my e-mail accounts - I've been sending everything the same way I've always done.
Anyway, at least now I know I can change e-mail account and send directly from Firefox, so thanks for that!