Firefox 4 was general released last week on all platforms including Mac OS, but several FF4 issues have been reported since. Some of them are fatal for Java applets (and consequently for 3DXplorer users on Mac). See this or this.
Mozilla will soon be releasing a corrective version.
In the meantime, we discourage Mac users to upgrade to FF4.
In case you have already upgraded to FF4, uninstall Firefox and reinstall an older version (3.6.17) from below: