Always open bookmarks in a new tab in the background

By default, opening a bookmark from the sidebar uses the current active
tab. The `browser.tabs.loadBookmarksInTabs` has been set tot `true` to
to always open bookmarks in a new tab in the background.

Closes GH-165

snowblocks/firefox/user.js

@@ -209,6 +209,12 @@ user_pref("browser.tabs.drawInTitlebar", true);
  */
 user_pref("browser.tabs.loadBookmarksInBackground", true);
 
+/*
+ * Always open bookmarks in a new tab in the background.
+ * By default, opening a bookmark from the sidebar uses the current active tab.
+ */
+user_pref("browser.tabs.loadBookmarksInTabs", true);
+
 /*
  * When you open a link in a new tab, switch to it immediately
  * Available within the UI under: