firefox-esr: fix build after 574a6d34d2

We're now using only newer versions that have ./configure in the root.
${pname} isn't the correct directory name for esr versions.

pkgs/applications/networking/browsers/firefox/default.nix

@@ -81,13 +81,9 @@ common = { pname, version, sha256 }: stdenv.mkDerivation rec {
 
   preConfigure =
     ''
+      configureScript="$(realpath ./configure)"
       mkdir ../objdir
       cd ../objdir
-      if [ -e ../${pname}-${version} ]; then
-        configureScript=../${pname}-${version}/configure
-      else
-        configureScript=../mozilla-*/configure
-      fi
     '';
 
   preInstall =