In reply to @mic92:nixos.dev
Just set a fixed port

Regarding opening the wrong firefox. It does this on my system but firefox isn't happy:

[pid 67440] execve("/run/wrappers/bin/xdg-open", ["xdg-open", "http://matrix.org/"], 0x1be9541f8000 /* 71 vars */) = -1 ENOENT (No such file or directory)
[pid 67440] execve("/home/andi/.nix-profile/bin/xdg-open", ["xdg-open", "http://matrix.org/"], 0x1be9541f8000 /* 71 vars */) = -1 ENOENT (No such file or directory)
[pid 67440] execve("/etc/profiles/per-user/andi/bin/xdg-open", ["xdg-open", "http://matrix.org/"], 0x1be9541f8000 /* 71 vars */) = -1 ENOENT (No such file or directory)
[pid 67440] execve("/nix/var/nix/profiles/default/bin/xdg-open", ["xdg-open", "http://matrix.org/"], 0x1be9541f8000 /* 71 vars */) = -1 ENOENT (No such file or directory)
[pid 67440] execve("/run/current-system/sw/bin/xdg-open", ["xdg-open", "http://matrix.org/"], 0x1be9541f8000 /* 71 vars */) = 0
strace: Process 67442 attached
[pid 67442] execve("/run/current-system/sw/bin/gio", ["gio", "help", "open"], 0x1328008 /* 72 vars */) = 0
[pid 67442] +++ exited with 0 +++
[pid 67440] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=67442, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
strace: Process 67443 attached
[pid 67443] execve("/run/current-system/sw/bin/gio", ["gio", "open", "http://matrix.org/"], 0x1328008 /* 72 vars */) = 0
strace: Process 67444 attached
strace: Process 67445 attached
strace: Process 67446 attached
strace: Process 67449 attached
strace: Process 67450 attached
[pid 67449] +++ exited with 0 +++
[pid 67443] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=67449, si_uid=1000, si_status=0, si_utime=0, si_stime=0} ---
[pid 67450] execve("/bin/sh", ["/bin/sh", "-e", "-u", "-c", "export GIO_LAUNCHED_DESKTOP_FILE"..., "sh", "firefox", "http://matrix.org/"], 0x1bc2400 /* 72 vars */) = 0
strace: Process 67451 attached
[pid 67446] +++ exited with 0 +++
[pid 67445] +++ exited with 0 +++
[pid 67451] +++ exited with 0 +++
[pid 67444] +++ exited with 0 +++
[pid 67443] +++ exited with 0 +++
[pid 67440] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=67443, si_uid=1000, si_status=0, si_utime=0, si_stime=2} ---
[pid 67440] +++ exited with 0 +++
[pid  5859] --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=67440, si_uid=1000, si_status=0, si_utime=0, si_stime=1} ---
[pid 67441] +++ exited with 0 +++
[pid 67450] execve("/run/current-system/sw/bin/firefox", ["firefox", "http://matrix.org/"], 0x109b008 /* 71 vars */) = 0

In reply to @andi:kack.it
I realized that element-desktop would always try to open a new firefox window but I didn't debug this any further. Would be great to get that fixed.