HTTPS/TLS is designed to be secure against adversaries with millions of dollars dedicated to attacking it. Why am I even dignifying systems that rely on the attacker's inability to use a command line, or write Perl code?
Have you reasoned through how much code it would take to make a "Firesheep" that worked against Javascript crypto implementations? It's not even a little bit hard.
JS encryption is certainly less secure than SSL, and may give a false sense of security to people who want to use it as a replacement.
On the other hand, it may be better than nothing.
Bruce Schneier wrote about why encrypting the video feed of predator drones was not as straightforward: http://www.schneier.com/blog/archives/2009/12/intercepting_p...
The command and control channel is, and always has been, encrypted -- because that's both more important and easier to manage. UAVs are flown by airmen sitting at comfortable desks on U.S. military bases, where key management is simpler. But the video feed is different. It needs to be available to all sorts of people, of varying nationalities and security clearances, on a variety of field terminals, in a variety of geographical areas, in all sorts of conditions -- with everything constantly changing. Key management in this environment would be a nightmare.