Images load and upload slower, they begin as grainy images, and clear up,slowly. I suppose the issue is the time involved. Thing is, the quality of My images is important to Me, I am not Geek enough to risk My best efforts on unproven systems. I may be a bit senile, I am damned sure old enough, but, whatever, Firefox just did not impress Me all that much.
I run Adaware, Spybot, and the MS spyware blocker, and Spyware Blaster. I run Bit Defender, and AVG, free, and it has been a long time since anything sneaked in.
Only reason Firefox ain't the target is that it is not yet worth aiming at, not that it is bullet proof.
Actually, I like how Firefox does that, since I'm on a dialup. I can get an idea of whether I really want the image or not, especially if it's a large file. With IE you have to wait until the image appears.
I've compared images saved with both browsers and personally see no difference.
Here's a good a test as any - color bars. Save it with both browsers and do a side by side comparison. (Why anyone would post color bars in a lossless format is another story).
http://www.unisatmobile.com/UniSat%20Color%20Bars.jpg
Addenda:
IE either waits until the image is completely downloaded, or it'll come line by line, or a few lines at a time, and I forgot to note that.
Still the image quality and file size is the same, regardless of the browser.