See also https://support.mozilla.com/en-US/questions/880899

Are there any errors shown in the error console when you try and do an attachment?

No, no errors are shown.

Does the issue still occur if you start Firefox in Safe Mode? http://support.mozilla.com/en-US/kb/Safe+Mode

Still does not work. I tried safe mode.

1) Do you know any way to get a demo account on MobileMe?

2) What type of file upload technology are they using? Flash uploader? Java uploader? A standard file upload dialog?

maybe you create and use an account for the app store made without a credit card:

http://support.apple.com/kb/ht2534

i have the same problem on a Mac after upgrading Firefox, no errors, just nothing is happening after clicking on the attachment button. I can add the attachment by drag and drop, but then the receiver tells me he or she received my message with a big red cross through the attachment.

It's exactly the same for my two PCs. One XP, the other Vista. Since the upgrade nothing attaches. All works with Safari and IE.

I filed a bug to get some more eyes on this -> https://bugzilla.mozilla.org/show_bug.cgi?id=695203