A couple days ago I ran into the issue of applications no longer starting
reliably on my openSUSE Leap 15.4 system. Launching applications like "glxinfo"
or "xlsclients" would fail with the error message `Maximum number of clients
reached:  unable to open display ":0"`.

In searching for the error message I came across
https://unix.stackexchange.com/questions/250920/debugging-maximum-number-of-clients-reached-unable-to-open-display-0.
Using the method described there to get the application using up the most x
client connections, I found that kmozillahelper was using up more than 170 of
them. Closing Firefox and one `killall kmozillahelper` later I could reliably
start applications again.

While my immediate problem is gone, it seems that there seems to be a leak of
kmozillahelper process instances when restoring a KDE session after having
logged out with an open Firefox. I.e., on third login after the killall, a `ps
aux | grep kmozillahelper`  will produce the following:

marix     5109  0.0  0.1 413872 44192 ?        Sl   11:10   0:00
/usr/lib/mozilla/kmozillahelper -session
10ca646469000165348143300000027740024_1653489712_189016
marix     5144  0.0  0.1 413872 44184 ?        Sl   11:10   0:00
/usr/lib/mozilla/kmozillahelper -session
10ca646469000165348930800000285810021_1653489712_189016
marix     6070  0.2  0.1 416140 46532 ?        Sl   11:10   0:00
/usr/lib/mozilla/kmozillahelper

Stopping Firefox will kill the process that does not have a -session argument.
However, if I logout without closing Firefox first, the next login will show
four kmozillahelper processes of which three have a with -session. The two
already existing in the current session, and a third one with a new sessionid.

As the session-id-less process instance is properly cleaned up if Firefox is
closed during a session, the problem can be worked around by closing Firefox
before logging out. However, this kind of contradicts the point of having
resumable sessions.