Try the following.

Close Firefox... open the Finder go the applications folder and select Firefox. Then press Cmd+i. There you select "open in 32 bit mode". Then open Firefox, click on this link (http://firefox-mac-pdf.s3.amazonaws.com/fx-quartz-pdf-1.2.0.xpi) and be happy.

From http://code.google.com/p/firefox-mac-pdf/issues/detail?id=189

Followed instructions but was not successful as the PlugIn was "not compatible" with Firefox 4

Ooops This suggestion works for me. I omitted to open Firefox in 32 bit mode, which is why it did not work the first time. Thanks

This suggestion works for me. I omitted to open Firefox in 32 bit mode, which is why it did not work the first time. Thanks