Extremely slow upload speeds with Mac OS X 10.8.2 and Firefox 18.0.1. Safari and Chrome works fine with Mac OS X and Firefox works fine on the Win 7 partition.

I am uploading some files to YouTube and other sides, each nominally about 50 MB using a dedicated 4 Mbit/s ethernet connection. When testing this connection using a PC and a Mac, the only configuration that doesn't work is Firefox with the Mac. Using any browser (including Firefox) on Windows 7 on a PC is fine, same with using Firefox, Chrome or IE on the Windows 7 partition of my MacBook Pro. Uploading on the MacBook Pro (running OS X 10.8.2) is fine with Safari and Chrome, but the files are taking 7x as long using Firefox (18.0.1) and Mac (OS X 10.8.2). What is unique about Firefox and Mountain Lion that is causing this? Thanks for any help that can be provided!