Firebug shows image downloads twice

On a website I'm working I have this problem, the Firebug timeline on Firefox shows that every single image is downloaded twice, this happens when I refresh the page usi开发者_如何学运维ng Ctrl-F5.

Chrome Developer Tools (Ctrl-Shift-I) shows that images are downloaded once as expected.

Please help me understand what this happens and how to prevent images from downloading twice on Firefox.

What do you mean by "Firebug timeline"?

When I visit those 2 pages, Firebug's Net panel shows the first page fetching 4 images (once each), and the second page fetching 1 image (once) plus "random.png".

I cannot see any problem, using CtrlF5 or not.

The response codes on a plain refresh (F5) are 304, and the cache seems to be working.

FF version: 3.6.13
Firebug Version: 1.6.2

Maybe there's a conflict with other add-ons and/or other Firebug plugins?