MatthewHSE - 11:16 pm on Apr 3, 2008 (gmt 0)
I found the about:config setting. It doesn't exist by default so you'll have to create it yourself. Go to about:config, right-click and choose New -> Boolean. Name it 'extensions.checkCompatibility' and set the value to FALSE. Then upgrade away and your extensions should be fine, provided you're coming from a 3.x version.