systemd docs about that limit says

Note that most process resource limits configured with these options are per-process

In reply to @nbp:mozilla.org
I recall this could be done on the command line, but I forgot the command since.

In reply to @vcunat:matrix.org
I really thought that setrlimit is per-process and I can't quickly find a reference.

       Set soft and hard limits on various resources for executed processes. See setrlimit(2) for

Set soft and hard limits on various resources for executed processes. See setrlimit(2) for [...]

In reply to @hexa:lossy.network
except for the test runner, which is stuck with 1024 for some reason

diff --git a/nixos/tests/firefox.nix b/nixos/tests/firefox.nix
index 3f9cea6662fb..6e311259a6eb 100644
--- a/nixos/tests/firefox.nix
+++ b/nixos/tests/firefox.nix
@@ -58,8 +58,19 @@ in
 
     };
 
+  extraPythonPackages = ps: with ps; [
+    psutil
+    types-psutil
+  ];
+
   testScript = ''
       from contextlib import contextmanager
+      import psutil
+
+
+      def sample_open_files():
+          for proc in psutil.process_iter():
+              machine.log(f"{proc.name()}={proc.num_fds()}")
 
 
       @contextmanager
@@ -95,6 +106,8 @@ in
 
       machine.wait_for_x()
 
+      sample_open_files()
+
       with subtest("Wait until Firefox has finished loading the Valgrind docs page"):
           machine.execute(
               "xterm -e '${firefoxPackage'.unwrapped.binaryName} file://${pkgs.valgrind.doc}/share/doc/valgrind/html/index.html' >&2 &"
@@ -102,6 +115,8 @@ in
           machine.wait_for_window("Valgrind")
           machine.sleep(40)
 
+      sample_open_files()
+
       with subtest("Check whether Firefox can play sound"):
           with record_audio(machine):
               machine.succeed(