<jsp:plugin type="applet"> generates type="application/x-java-applet;" instead of type = "application/x-java-applet". Firefox with Java-plugin cannot display Java applets because ";" is added.
Note that the ';' is expected in some circumstances. Eg: type="application/x-java-applet;version=1.5.0" That said, the ';' should really only be present if there is a following parameter so I have patched trunk to that effect and proposed the change for 5.5.x and 6.0.x
This has been fixed in 6.0.x and will be included in 6.0.20 onwards.
This has been fixed in 5.5.x and will be included in 5.5.28 onwards.