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.

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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?