If you like using Firefox 3, but you miss using your extensions which are not compatible with Firefox 3, there is something you can try. Go to the address bar and type: about:config once it shows the list of configuration preferences, right click on the list and select New -> Boolean from the menu. This will pop up a window that asks for the name of the new preference; enter extensions.checkCompatibility and set the value to false.

Once you restart your browser, it will no longer check for plug-in compatibility when Firefox 3 starts. I've found that even though most of my plug-ins are not "technically compatible", they still work in Firefox 3 without any apparent issues once you can get by the compatibility check.