I'm on the latest public version of OS X, and latest public version of Firefox.
After you click on the zoom button on the left floating menu, and then immediately after you scroll over to try to select a zoom level, the dropdown disappears.
If you keep your mouse still (in this example) on 120% at the top, the dropdown remains visible, but as soon as you move from (in this example) the 120% option at the top (and, for example, try to move down to select another option) it disappears. It makes zooming to a specific percentage quite difficult.
After you click on the zoom button on the left floating menu, and then immediately after you scroll over to try to select a zoom level, the dropdown disappears.
If you keep your mouse still (in this example) on 120% at the top, the dropdown remains visible, but as soon as you move from (in this example) the 120% option at the top (and, for example, try to move down to select another option) it disappears. It makes zooming to a specific percentage quite difficult.