tor-browser-bundle-bin: optionally disable multiprocess support

Multiprocess tabs always crash, as first reported by the issue mentioned
below.  It is now consistently reproducible both on NixOS and non-NixOS
for me, so I've decided to add a toggle to conveniently disable
multiprocess support as a work-around.

Closes https://github.com/NixOS/nixpkgs/issues/27759 but does
not really fix the underlying problem ...

pkgs/applications/networking/browsers/tor-browser-bundle-bin/default.nix

@@ -44,6 +44,10 @@
 , hicolor_icon_theme
 , shared_mime_info
 
+# Whether to disable multiprocess support to work around crashing tabs
+# TODO: fix the underlying problem instead of this terrible work-around
+, disableContentSandbox ? true
+
 # Extra preferences
 , extraPrefs ? ""
 }:
@@ -210,6 +214,10 @@ stdenv.mkDerivation rec {
     lockPref("extensions.torlauncher.control_port_use_ipc", true);
     lockPref("extensions.torlauncher.socks_port_use_ipc", true);
 
+    // Optionally disable multiprocess support.  We always set this to ensure that
+    // toggling the pref takes effect.
+    lockPref("browser.tabs.remote.autostart.2", ${if disableContentSandbox then "false" else "true"});
+
     ${optionalString (extraPrefs != "") ''
       ${extraPrefs}
     ''}