1. Install the Firefox addon named User Agent Switcher88.

2. Once installed, go to Tools >> User Agent Switcher >> Options >> Options.
The User Agents Switcher Options window is displayed.

3. Select User Agents from the list in the leftmost part of your screen.
4. Click Add to add a new user agent and type these details in the resulting window:
a. In the Description box type : MEGAUPLOAD
b. In the User Agent box type : Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; Alexa Toolbar)

5. Click OK twice to close the two windows and finish adding the new user agent.

So how do you use it from here? Just before you download a file from Megaupload, go to
Tools >> User Agent Switcher >> MEGAUPLOAD.
You can always set the user agent back to default after your download is complete.