On about 2022-01-24, I updated firefox esr from 78.11.0esr-1 to
91.5.0esr-1.  A new misbehavior appeared.

Starting with a freshly launched browser, if I visit four pages in
succession without backing up (three seems not to be enough to trigger
the problem), then use Alt-Left to return to the new window start page,
the history list gets disordered in a way that is hard to describe.
If one holds the back button, one should see pages to which one could
go backwards, but if there _are_ no older pages, then the button should
be greyed out.  But when this problem happens, there will still be
pages in the "older" list, and the button will not be greyed out.
It's usually possible to use the mouse and the back button to get to
the start page, but _not_ usually possible to use Alt-Left to do so,
as the latter seems to end up looping through a set of entries in
the history list.

The 91.5.1esr-1 update was applied today, and does not seem to change
the behavior.