Is there any difference if you use a private browser window in Firefox? That might indicate a cache or cookie issue. Sometimes a problem is due to changes on eBay's end.
If you greatly prefer Firefox to Chrome, you might try installing a parallel version such as Firefox Developer Edition alongside your current Firefox version. That can allow you to have a second Firefox version with its own profile, cache and cookies, so that you can use it to try alternative settings without having to change settings in your primary browser or having to rely on Chrome.
If you sign in to the same Mozilla account, you can share bookmarks and extensions between the browser versions -- not sure if other settings are shared as well; you may want to sign out of your Mozilla account in one browser to experiment if you are concerned about that, once you have things set up and working for you.
If the problem really is on Firefox's end, both versions may have the same problem, though.
