It’s cool that the EFF is offering a Firefox extension to use SSL where it’s available. I prefer to encrypt things too, but really, that’s the least of your problems.
As I am sure you know, HTTPS simply encrypts data between your computer and the web server, while it’s in transit. It does nothing to ensure that your data is handled correctly at the other end.
When you hear about credit card or other privacy breaches, it’s usually due to mishandling — databases that get into the wrong hands, or software flaws that reveal information. SSL can’t help you there.
Using SSL by default is still a good idea. But it’s like locking the car doors on the way to drop off your kid at day care. Those doors locks are fine, but it’s the day care you need to trust.