--- a/mobile/android/geckoview/src/main/java/org/mozilla/gecko/permissions/PermissionBlock.java
+++ b/mobile/android/geckoview/src/main/java/org/mozilla/gecko/permissions/PermissionBlock.java
@@ -15,16 +15,17 @@ import android.support.annotation.NonNul
  * Helper class to run code blocks depending on whether a user has granted or denied certain runtime permissions.
  */
 public class PermissionBlock {
     private final PermissionsHelper helper;
 
     private Context context;
     private String[] permissions;
     private boolean onUIThread;
+    private boolean onBackgroundThread;
     private Runnable onPermissionsGranted;
     private Runnable onPermissionsDenied;
     private boolean doNotPrompt;
 
     /* package-private */ PermissionBlock(Context context, PermissionsHelper helper) {
         this.context = context;
         this.helper = helper;
     }
@@ -41,17 +42,27 @@ public class PermissionBlock {
      * Execute all callbacks on the UI thread.
      */
     public PermissionBlock onUIThread() {
         this.onUIThread = true;
         return this;
     }
 
     /**
+     * Execute all callbacks on the background thread.
+     */
+    public PermissionBlock onBackgroundThread() {
+        this.onBackgroundThread = true;
+        return this;
+    }
+
+    /**
      * Do not prompt the user to accept the permission if it has not been granted yet.
+     * This also guarantees that the callback will run on the current thread if no callback
+     * thread has been explicitly specified.
      */
     public PermissionBlock doNotPrompt() {
         doNotPrompt = true;
         return this;
     }
 
     /**
      * If the condition is true then do not prompt the user to accept the permission if it has not
@@ -111,18 +122,24 @@ public class PermissionBlock {
         executeRunnable(onPermissionsDenied);
     }
 
     private void executeRunnable(Runnable runnable) {
         if (runnable == null) {
             return;
         }
 
-        if (onUIThread) {
+        if (onUIThread && onBackgroundThread) {
+            throw new IllegalStateException("Cannot run callback on more than one thread");
+        }
+
+        if (onUIThread && !ThreadUtils.isOnUiThread()) {
             ThreadUtils.postToUiThread(runnable);
+        } else if (onBackgroundThread && !ThreadUtils.isOnBackgroundThread()) {
+            ThreadUtils.postToBackgroundThread(runnable);
         } else {
             runnable.run();
         }
     }
 
     /* package-private */ String[] getPermissions() {
         return permissions;
     }

--- a/mobile/android/geckoview/src/main/java/org/mozilla/gecko/permissions/Permissions.java
+++ b/mobile/android/geckoview/src/main/java/org/mozilla/gecko/permissions/Permissions.java
@@ -29,18 +29,21 @@ import java.util.concurrent.FutureTask;
  *    Permissions.from(activity)
  *               .withPermissions(Manifest.permission.WRITE_EXTERNAL_STORAGE)
  *               .onUiThread()
  *               .andFallback(onPermissionDenied())
  *               .run(onPermissionGranted())
  *
  * This example will run the runnable returned by onPermissionGranted() if the WRITE_EXTERNAL_STORAGE permission is
  * already granted. Otherwise it will prompt the user and run the runnable returned by onPermissionGranted() or
- * onPermissionDenied() depending on whether the user accepted or not. If onUiThread() is specified then all callbacks
- * will be run on the UI thread.
+ * onPermissionDenied() depending on whether the user accepted or not.
+ * <p>
+ * If onUiThread()/onBackgroundThread() is specified, all callbacks will be run on the respective
+ * thread. Otherwise, the callback may run on the current thread, but will switch to the UI thread
+ * if a permissions prompt is required.
  */
 public class Permissions {
     private static final Queue<PermissionBlock> waiting = new LinkedList<>();
     private static final Queue<PermissionBlock> prompt = new LinkedList<>();
 
     private static PermissionsHelper permissionHelper = new PermissionsHelper();
 
     /**

--- a/mobile/android/tests/background/junit4/src/org/mozilla/gecko/permissions/TestPermissions.java
+++ b/mobile/android/tests/background/junit4/src/org/mozilla/gecko/permissions/TestPermissions.java
@@ -218,16 +218,30 @@ public class TestPermissions {
                 .withPermissions(Manifest.permission.WRITE_EXTERNAL_STORAGE)
                 .doNotPromptIf(true)
                 .andFallback(mock(Runnable.class))
                 .run(mock(Runnable.class));
 
         verify(helper, never()).prompt(anyActivity(), any(String[].class));
     }
 
+    @Test(expected = IllegalStateException.class)
+    public void testCannotRunCallbackOnMultipleThreads() {
+        PermissionsHelper helper = mockDenyingHelper();
+        Permissions.setPermissionHelper(helper);
+
+        Permissions.from(mockActivity())
+                .onBackgroundThread()
+                .onUIThread()
+                .withPermissions(Manifest.permission.WRITE_EXTERNAL_STORAGE)
+                .doNotPromptIf(true)
+                .andFallback(mock(Runnable.class))
+                .run(mock(Runnable.class));
+    }
+
     private Activity mockActivity() {
         return mock(Activity.class);
     }
 
     private PermissionsHelper mockGrantingHelper() {
         PermissionsHelper helper = mock(PermissionsHelper.class);
         doReturn(true).when(helper).hasPermissions(any(Context.class), anyPermissions());
         return helper;