Something similar is happening to me, also on Mac, also in Firefox and Safari. The page jumps aggressively whenever I type into any item specific field instead of clicking a box. Reorients the open field immediately down to the bottommost visible spot in whatever browser window I'm in. It's infuratingly distracting, jarring, and totally breaks the flow. Then after that happens the window creates almost an infinite scroll and relocates the photos so far up you'd have to scroll for literally a minute to find them (or just hit Home of course on the keyboard, but still **bleep**)
I spoke to an Ebay person on the phone today about it because it was driving me crazy. She had me open an incognito window in Chrome and see if it was happening there. I don't use Chrome (on principle but that's another matter entirely) but I did have it installed still. So I tried that - and the jumping around stopped. But still it forces me into the Chrome environment which I don't like. She said to use Chrome for a few days and they're working on fixing it, because I'm not the only one who reported.
So try Chrome if you can stand it. Otherwise just wait, they'll fix it "soon" whatever the hell that means.
