I think so. I'm sure mozilla will not release the latest version if it's not compatible with plug-ins.
Exactly, it can support the old plugins, and it also add some new add-ons. And the big change is its interface. It makes the tabs on the top, and all the files manu in only one button.
firefox 4.02b has been released and its good version cuz i m using it but the main drawback is most of the addons are not working with even it does not support alexa toolbar.........