Symfony2 request called twice - symfony

Question

I have a problem with Symfony2 duplicating requests.
The example controller below should just return a transparent gif. However, when the URL is requested there are two 'hits' to the request and doSomething() is called twice instead of once.
// simple action in a controller, ends up being called twice
public function testAction() {
doSomething();
$response = new Response(base64_decode('R0lGODlhAQABAIAAAOrq6gAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw=='));
$response->headers->set('Content-Type', 'image/gif');
return $response;
}
If I change the response line so it sets a blank response then this stops happening (i.e. doSomething() is only called once):
// with a blank response it is only called once
public function testAction() {
doSomething();
$response = new Response('');
$response->headers->set('Content-Type', 'image/gif');
return $response;
}
If I remove the headers line then it's only called once:
// without the Content-Type header it is only called once
public function testAction() {
doSomething();
$response = new Response(base64_decode('R0lGODlhAQABAIAAAOrq6gAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw=='));
return $response;
}
Finally (here's the punchline)... if I change the Content-Type to text/html (or text/css) then it is only called once!
// if a text content type is used, it is only called once
public function testAction() {
doSomething();
$response = new Response(base64_decode('R0lGODlhAQABAIAAAOrq6gAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw=='));
$response->headers->set('Content-Type', 'text/html');
return $response;
}
But with real content and the proper headers it is called twice (and two identical looking entries appear in the web profiler). The same problem also happens if a php template is used to render the content (I haven't tried with twig).
What could be causing this and how can it be prevented (or worked around)?
Edit: So, in summary, it was a Firefox bug and nothing to do with Symfony at all.