Update webdriver.xpi to v2.48.2 with new /hub context

Our old webdriver.xpi was from v2.45 of the selenium server jar. In
SeleniumHQ/selenium@e581237f8288, which occured between v2.45 and
v2.48.2, the prefix for the webdriver context changed to strictly `/hub`
from `.*/hub`, so we also update the context prefix in Firefox.pm
correspondingly.

lib/Selenium/Firefox.pm

@@ -79,6 +79,11 @@ has '_binary_args' => (
     }
 );
 
+has '+wd_context_prefix' => (
+    is => 'ro',
+    default => sub { '/hub' }
+);
+
 with 'Selenium::CanStartBinary';
 
 =attr startup_timeout

lib/Selenium/Firefox/webdriver_prefs.json

@@ -2,15 +2,18 @@
   "frozen": {
     "app.update.auto": false,
     "app.update.enabled": false,
+    "browser.displayedE10SNotice": 4,
     "browser.download.manager.showWhenStarting": false,
     "browser.EULA.override": true,
     "browser.EULA.3.accepted": true,
     "browser.link.open_external": 2,
     "browser.link.open_newwindow": 2,
     "browser.offline": false,
+    "browser.reader.detectedFirstArticle": true,
     "browser.safebrowsing.enabled": false,
     "browser.safebrowsing.malware.enabled": false,
     "browser.search.update": false,
+    "browser.selfsupport.url" : "",
     "browser.sessionstore.resume_from_crash": false,
     "browser.shell.checkDefaultBrowser": false,
     "browser.tabs.warnOnClose": false,
@@ -28,6 +31,7 @@
     "extensions.logging.enabled": true,
     "extensions.update.enabled": false,
     "extensions.update.notifyUser": false,
+    "javascript.enabled": true,
     "network.manage-offline-status": false,
     "network.http.phishy-userpass-length": 255,
     "offline-apps.allow_by_default": true,
@@ -48,7 +52,8 @@
     "toolkit.networkmanager.disable": true,
     "toolkit.telemetry.prompted": 2,
     "toolkit.telemetry.enabled": false,
-    "toolkit.telemetry.rejected": true
+    "toolkit.telemetry.rejected": true,
+    "xpinstall.signatures.required": false
   },
   "mutable": {
     "browser.dom.window.dump.enabled": true,