Hello ssh maintainers,
first of all, I'm not sure if this is a bug in the client or server. I
was prompted and selected "both", but then reportbug told me the
package ssh does not exist. So please reassign as appropriate.

This is a regression compared to bookwork, from which I upgraded this
machine a few weeks ago.

Under X I run "ssh -X localuser@localhost" where localuser is a
different uid than my ordinary one.

Once logged in I launch
firefox

Firefox takes long to start (but this happened in bookworm as well).
In Bookworm I was then able to use firefox as expected.

In Testing Firefox works for a few seconds, then "pauses" for ~ 5
seconds and then works again for a few seconds, "pauses" for ~ 5
seconds and so on. Note that keyboard input is buffered, so if I e.g.
type an URL during this pause, the keystrokes are delivered after the
pause.

To narrow this down:
firefox without ssh does not exhibit this problem.

/usr/lib/x86_64-linux-gnu/libexec/kf5/kdesu -u localuser /opt/firefox/firefox

also does not exhibit this problem.

So something in ssh (client or server) must have changed to cause
these delays.

I ran the ssh session in verbose mode (-vvv), where I run the above
command, launched firefox, hit one or two of these delays, closed
firefox and logged out. The log is attached.

If I should try something else or if you need other/further logs,
simply tell me so that I can provde them.