Table with pinned column containing row headers with Firefox

Zaitchik, Alan

I tested with NVDA 2021.2 and FIrefox (93.0) and found that a table with a pinned column in column 1 that contains row header cells does not support navigation up and down column 1 using table navigation commands (Control+Alt+arrow). The row headers are read out correctly, however, when navigating up and down other columns. If I remove the row header cells, or move them to column 2, then navigation of the pinned column works well -- but of course my row header cells can now scroll out of view. NVDA works fine with Chrome and this table.
Note that the pinning is done with CSS but the table is otherwise unexceptional, just some column headers (using scope="col") and a sort button in one column header cell. No Javascript errors or the like get thrown on load. And as I said, problems are gone when the row headers are changed to data cells. The html validates as correct.
The pinning is through a css class 
.headcol {
  position: absolute;
  width: 5em;
  left: 0;
  top: auto;
  border-top-width: 1px;
  margin-top: -1px;
which is assigned in every row to the cell in column 1.
Here is a code pen with the problem case: 
Is this a known problem? Is it the css?