Make Wireplumber route Firefox' voice output to internal ALSA output

41-firefox-callback.lua

stream_defaults.rules = {
    {
        matches = {
            {
                { "application.name", "matches", "Firefox" },
                { "media.name", "matches", "AudioCallbackDriver" },
                { "media.class", "matches", "Stream/Output/Audio" },
            }
        },
        apply_properties = {
            ["target"] = "alsa_output.pci-0000_06_00.6.analog-stereo",
            ["state.save-target"] = false,
        }
    }
}

README.md

This makes Wireplumber (the new audio routing engine for Pipewire, used by default in Ubuntu 22.10) route all voice streams from Firefox, e.g. during video calls (identified by their media.name being "AudioCallbackDriver") to another sink than the default (e.g. your headphones).

1. Copy the 41-firefox-callback.lua to ~/.config/wireplumber/main.lua.d/ and adapt the target value to your needs (pactl list sinks)
2. Copy /usr/share/wireplumber/scripts/restore-stream.lua to ~/.config/wireplumber/scripts/restore-stream.lua
3. Apply the restore-stream.lua.patch to that copy

The patch is neccessary because by default Wireplumber would ignore the target stream prop and always use the one from the file (~/.local/state/wireplumber/restore-stream). And even if it did use the stream prop, it would then save it to the restore-stream, but only matching Firefox and hence errornously redirecting e.g. music output from Firefox to your headphones, too.

restore-stream.lua.patch

--- /usr/share/wireplumber/scripts/restore-stream.lua	2022-10-04 15:25:09.000000000 +0200
+++ ~/.config/wireplumber/scripts/restore-stream.lua	2022-11-03 14:55:39.062548314 +0100
@@ -139,7 +139,7 @@
   local stream_props = node.properties
   rulesApplyProperties(stream_props)
 
-  if stream_props["state.restore-target"] == false then
+  if stream_props["state.restore-target"] == false or stream_props["state.save-target"] == false then
     return
   end
 
@@ -353,7 +353,10 @@
   end
 
   if config_restore_target and stream_props["state.restore-target"] ~= false then
-    local str = state_table[key_base .. ":target"]
+    local str = stream_props["target"]
+    if not str then
+        str = state_table[key_base .. ":target"]
+    end
     if str then
       restoreTarget(node, str)
     end

Have you considered a PR into https://gitlab.freedesktop.org/pipewire/wireplumber ?