I'm seeing better results in Firefox on macOS but I can see it isn't quite as optimal on Windows.
In other browsers, including Chrome and Safari it works perfectly.
To be honest, I'm not entirely sure we want to be making any changes here. We don't do anything fancy with the sticky header - it's literally just vanilla CSS position: sticky
in modern browsers and the majority of them seem to be accounting for it automatically, so Firefox should be too.
If anything, this will be a bug that should be fixed in Firefox, so might be worth a report on Bugzilla.