User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.14; rv:65.0) Gecko/20100101 Firefox/65.0
Steps to reproduce:
In the attached file, I have an input range (slider) and a combobox (select). They are sync'd using jQuery so that manipulating one also changes the other. I first move the slider at which point the combobox gets the focus. I then use the up/down arrow keys an observe how the combobox selection changes. Then I move the slider some distance away and try the up/down arrow keys again. Again I observe the change in selection.
The first time (see above) I use the up/down arrow keys, the selection changes by one up or down as appropriate. But the second time (after using the slider again - see above) I use the up/down arrow keys the selection jumps, reverting to one above or below the prior selection made with the arrow keys initially. The selection provided via the slider is ignored.
I would expect the up/down arrow keys to always move one up/down from the current selection, however that selection was made. This is how Chrome, Safari, Opera and MS Edge appear to work.