it's just a limitation of killall/pstree it seems. Per man page

>        If processes change their name, killall may not be able to match them correctly.

that's what I see:
$ ps -o cmd,comm 7691
CMD                         COMMAND
/usr/lib64/firefox/firefox  GeckoMain

$ pgrep -f firefox
7691
7695
7807
7883
7932

$ pstree --hide-threads --show-pid 7691
GeckoMain(7691)─┬─Privileged Cont(7932)
                ├─Socket Process(7807)
                ├─WebExtensions(7883)
                └─chrome-gnome-sh(7922)

While ps usually display the "CMD", pstree/killall uses the "COMMAND".
Both should work:

pkill -f firefox

or

killall GeckoMain

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lifetime, sometimes those efforts are overtaken by events. Often a 
more recent Fedora release includes newer upstream software that fixes 
bugs or makes them obsolete.

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plan to fix it in a currently maintained version, simply change the 'version' 
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version prior this bug is closed as described in the policy above.

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I will check this here asap. keeping the br open for now.

As for Fedora 34:

[user@hostname ~]$ pstree -up| grep firefox
[user@hostname ~]$ killall firefox
firefox: Kein Prozess gefunden
[user@hostname ~]$ killall GeckoMain
[user@hostname ~]$ uname -a
Linux hostname 5.14.18-200.fc34.x86_64 #1 SMP Fri Nov 12 16:48:10 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
[1]+  Beendet                 firefox

This message is a reminder that Fedora Linux 34 is nearing its end of life.
Fedora will stop maintaining and issuing updates for Fedora Linux 34 on 2022-06-07.
It is Fedora's policy to close all bug reports from releases that are no longer
maintained. At that time this bug will be closed as EOL if it remains open with a
'version' of '34'.

Package Maintainer: If you wish for this bug to remain open because you
plan to fix it in a currently maintained version, change the 'version' 
to a later Fedora Linux version.

Thank you for reporting this issue and we are sorry that we were not 
able to fix it before Fedora Linux 34 is end of life. If you would still like 
to see this bug fixed and are able to reproduce it against a later version 
of Fedora Linux, you are encouraged to change the 'version' to a later version
prior to this bug being closed.

Fedora Linux 34 entered end-of-life (EOL) status on 2022-06-07.

Fedora Linux 34 is no longer maintained, which means that it
will not receive any further security or bug fix updates. As a result we
are closing this bug.

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The needinfo request[s] on this closed bug have been removed as they have been unresolved for 365 days