Install MrTech Toolkit Alpha (aka Local Install) to make it compatible.

Tools > Addons
Tools > Make all compatible
or you can right click the extension which FireFox marks not compatible and select "Make Compatible".

You also need to create this entry in about:config and set it to false.

Be aware that forcing an extension to work with Firefox 3 in this way may have undesired results. Do so at your own risk.