ok, in this case please try to reset firefox & see if this is making a difference:
I have the same problem - For me it happens on secure sites. The lock hides the java plugin icon. I can see the java red plugin icon just fine on pages that are not https:/encrypted.
Since the person above is shopping and downloading, I suspect that he is also trying to access features of a encrypted sites.
It's quite frustrating.
SOLUTION--- Bizarrely, apparently Java was not installed at all on my PC. (I am running Windows 8 on a new HP Pavilon g7.)
I hope this works for you!