Switch to a prebuilt legacy firefox xpi

This means we can upgrade closure without needing
to ensure that this XPI remains the same. It was
built from:

```
git checkout 684c726
./buckw build //javascript/firefox-driver:webdriver
```

And the output of that copied to the right place.

dotnet/src/webdriver/BUCK

@@ -173,7 +173,7 @@ genrule(
 genrule(
   name = 'webdriver_deps',
   srcs = [
-    '//javascript/firefox-driver:webdriver_prefs',
+    '//third_party/js/selenium:webdriver_prefs',
     '//javascript/webdriver/atoms:get-attribute',
     '//javascript/atoms/fragments:is-displayed'
   ],

java/client/src/org/openqa/selenium/firefox/BUCK

@@ -30,6 +30,6 @@ java_library(name = 'firefox',
 )
 
 export_file(name = 'prefs',
-  src = '//javascript/firefox-driver:webdriver_prefs',
+  src = '//third_party/js/selenium:webdriver_prefs',
   out = 'webdriver_prefs.json',
 )

java/client/src/org/openqa/selenium/firefox/xpi/BUCK

@@ -33,7 +33,7 @@ java_library(name = 'firefox-xpi',
 )
 
 export_file(name = 'webdriver.xpi',
-  src = '//javascript/firefox-driver:webdriver',
+  src = '//third_party/js/selenium:webdriver',
 )
 
 export_file(