I have created several custom tabs. Now I want to move the "Metadata" tab to end.

I am pretty sure this can be done by removing it then adding it in again at the end, ie:

 $f->removeFieldFromTab("Root.Content","Metadata");
$f->addFieldToTab("Root.Content","Metadata");

Howard, your idea does not work.

The first line successfully removes the Metadata tab, but when I add the second tab I get an error:

Fatal error: Call to a member function Name() on a non-object in FieldSet.php on line 393

It seems that it cannot be added again because it was removed.

This worked for me:

$metadataTab = $fields->findOrMakeTab('Root.Content.Metadata');
$fields->removeByName('Metadata');
$fields->addFieldToTab('Root.Content', $metadataTab);