Upload Failures with FireFox Manual Proxy Server Configuration
Customers who require an HTTP proxy for Internet access may experience failed uploads when using FireFox with manual proxy configuration.
This is due to the Adobe Flash Plugin not properly using proxy settings, as described in FireFox Bug 562880. The recommended work-around is to configure the proxy in the settings for the Operating System, then configure FireFox to use System Proxy Settings.
Using a Proxy server with Mac
Using a Proxy Server with Windows
Configuring FireFox to use System Proxy Settings