No, it isn't. It's actually harder. To intercept a stream, you need only capture a few packets. To passively observe it, you have to position yourself in a place that sees all the packets.
Edited to clarify since apparently I can't reply to a child comment: There are tools that require next-to-no technical expertise to passively observe HTTP sessions. There are no such tools that I'm aware of to execute an HTTP MITM without control of the network infrastructure.
I think what the original poster meant was basically just "in a situation where you absolutely, definitely, cannot have SSL, javascript crypto is an iota better than nothing, because at least it breaks Firesheep and Wireshark".
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.
Despite what Hollywood says, using a command line does not make you a super leet hacker. If your security system can be broken with a command line, then I doubt you care that much about the data you're securing. Just put the data in plain text and call a spade a spade.