I have run into this too, it surprised me so much that i thought maybe it was just a problem with my account, but apparently others are experiencing the same problem. the only way around it that i have discovered thus far is using your iCloud mail with your Mail on your mac. then you can send attachments flawlessly. hopefully it will become possible to do it the other way soon.

This is not only happening with iCloud but also with MobileMe (iCloud's precursor). I had Lion reinstalled (today) thinking that may be the cause but even with a blank os it hasn't fixed it. I then sought assistance with AppleSupport/MobileMe but it could not be resolved. It appears to me (and I am not a tech-head) that the current version of Firefox does not support the changes going on with MobileMe/iCloud. I think Firefox have to provide the fix. I much prefer to use Firefox over Safari but at the moment, if I want to attach attachments, I am forced to use Safari (note: I do not download my mail using Mail as I prefer to access my mail from the net).

You don't have to disable any addons or extentions!!!You guys are not going to believe this I figured out the problem it's really dumb...ok here goes the attachment uploader loads seperate so I got to thinking it's a popup so I went to options content and under block pop ups there is an option exceptions click on that and in the box type firefox.com and hit allow pow its fixed!!!!! :-)