Same here on Firefox 157.0 and macOS 27.0.1, including in a new private window.
I was able to narrow down the workaround mentioned above (OpenAI devs, pay attention):
Focusing .thread-scroll-container alone restores ⌘↑/⌘↓. Adding tabindex="0" isn’t necessary.
document.querySelector(".thread-scroll-container")
.focus({ preventScroll: true });
Clicking ordinary message text gives focus to the outer main element instead. Returning focus there makes the shortcuts stop working again. Adding tabindex="0" while keeping focus on main doesn’t help.
I experience the same issue in macOS’s ChatGPT.app by the way, version 26.930.31730.