Starting today users on Firefox 3.5 (or older versions) will be offered a direct upgrade to Firefox 3.6. Note: The upgrade offer will not appear until there has been at least 10 minutes without any keyboard activity. The offer can also be access via the Help Menu and selecting Check For Updates…

Users can able to choose to:

• Wait 24 hours (“Ask Later”)
• Decline the offer (“No Thanks”)
• Accept the free upgrade (“Get the New Version”)

Source: Mozilla Developers News