firefox-esr: Fix name

The Firefox wrapped called itself "firefox" rather than "firefox-esr".

Also eliminate a use of splitString which is evil and should never be
used.

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

@@ -122,6 +122,7 @@ common = { pname, version, sha256 }: stdenv.mkDerivation rec {
   passthru = {
     inherit gtk nspr version;
     isFirefox3Like = true;
+    browserName = pname;
   };
 };
 

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

@@ -9,8 +9,8 @@
 }:
 
 ## configurability of the wrapper itself
-browser :
-{ browserName ? (lib.head (lib.splitString "-" browser.name))  # name of the executable
+browser:
+{ browserName ? browser.browserName or (builtins.parseDrvName browser.name).name
 , name ? (browserName + "-" + (builtins.parseDrvName browser.name).version)
 , desktopName ? # browserName with first letter capitalized
   (lib.toUpper (lib.substring 0 1 browserName) + lib.substring 1 (-1) browserName)