abp:subscribe fixes

Test a[href^="abp:"], since the protocol probably not used for anything
else, also "a", because ABP checks only anchors, and these links are made
only for ABP.

Also, the event target is not always the link, so at least some parents
should be tested as well.

platform/firefox/vapi-background.js

@@ -455,11 +455,9 @@ vAPI.tabs.getTabId = function(target) {
     if ( vAPI.fennec ) {
         if ( target.browser ) {
             // target is a tab
-            return target.browser.loadContext.DOMWindowID;
+            target = target.browser;
         }
         return target.loadContext.DOMWindowID;
-
-        return -1;
     }
 
     if ( target.linkedPanel ) {