dropbox: install firefox-bin in FHS env

To sign in to dropbox, a browser must be available in the FHS env. We cannot
ensure that the user's browser of choice is available, so we provide Firefox as
a default.

Resolves: #31667

pkgs/applications/networking/dropbox/default.nix

@@ -35,7 +35,8 @@ buildFHSUserEnv {
   targetPkgs = pkgs: with pkgs; with xlibs; [
     libICE libSM libX11 libXcomposite libXdamage libXext libXfixes libXrender
     libXxf86vm libxcb xkeyboardconfig
-    curl dbus fontconfig freetype gcc glib gnutar libxml2 libxslt procps zlib
+    curl dbus firefox-bin fontconfig freetype gcc glib gnutar libxml2 libxslt
+    procps zlib
   ];
 
   extraInstallCommands = ''
@@ -44,6 +45,8 @@ buildFHSUserEnv {
   '';
 
   runScript = writeScript "install-and-start-dropbox" ''
+    export BROWSER=firefox
+
     set -e
 
     do_install=