Image / Media Uploader Forces Login Screen in Firefox and Opera

Hello! A few days ago I updated my site http://frozen-flower.net to version 2.7 of wordpress. I think I was using 2.5 before this. The flash media uploader worked fine for me then, but now I am experiencing this behaviour:

I click Select Files, choose my images, and press okay. The uploader appears to be working, it uploads the images, then says “crunching”. Instead of showing the options for the image below when it’s done, it shows the WordPress Login screen. If I try multiple images, it will show a Login screen in place of each and every image option panel.

I’ve followed the advice from the sticky thread. I’ve disabled all my plugins while trying out different fixes. I’ve altered my htaccess files based on advice from several threads. I’ve deleted the admin files and reuploaded them. I’ve cleared my cookies and cache several times, rebooted my browser, etc. I’ve updated to the newest flash player. I’ve tried small images, large images, images one at a time, images in groups.

The strange thing is that the media uploader works fine in Internet Explorer only! Shocking! I’ve tested it under Opera, Firefox, and Google Chrome and those three will not run the media uploader on my WordPress install. But since Internet Explorer is not my browser of choice, I’d rather get it work everywhere. [I kind of giggled when I read one of the answers on the support thread was “Stop using Internet Explorer already” since it’s what I’ve been forced to use to upload images on mass lately. ;D]

I’ve been stalking all the threads I could find with this issue, and it seems that the only ones that are listed as “resolved” have had the person give up on the flash uploader and start using the browser uploader instead. For me, this isn’t the solution I want. I want to be able to upload photo sets in my blog entries, so uploading the images one at a time is not the best use of time. 🙁

Does anyone have any new ideas that they think I should try? Or are you experiencing this problem too? I appreciate any insight on the situation! Thanks!