...then could an sslsniff attack be successfull against me?
Could the attacker get informations because the sslsniff degraded the connection from HTTPS to HTTP?
Let me add that your description does not exactly fit to how "sslsniff" works. SSLsniff acts more like a proxy intercepting SSL traffic, instead of simply degrading your connection. In other words: SSLsniff messes with the certificates more than with the HTTPS connection itself.
This means your "addon" might actually think everything is fine, while in fact your connection is as unsecure as it can get and you would not know about it.
Or I am "fully safe" from these kind of attacks when using HTTPS Everywhere?
Never expect a "browser addon" to provide "system protection".
To give you a human perspective: You wouldn't send boyscouts to war either, would you? Of course not, since they would not be fit and able enough to handle the job. Well, neither is a simple browser addon when it comes to SSL security.
Think about it: anyone messing with your browser could mess with the addon. In a worst-case scenario, the addon could be modified to not even warn you that you've "lost" your secure ssl connection while browsing and you would not know about it. Any virus, malware or even new browser exploit would be able (depending on it's individual malicious purpose) to mess with your system... including the software on your system (which includes your web-browser and it's addons and plugins).
UPDATE: and what happens if I have the domain whitelisted in HTTPS Everywhere? [xml files could be created]. So the domain would be only available via HTTPS.
Getting back to your case and example: you're worrying about the HTTPS connection. Your browser might use that connection, but it can not be sure the data it's sending or receiving is not being intercepted and neither can any addon. Actually, detecting such an "interception" can already be pretty hard even when you're using "tools of the trade". Your webbrowser and all the addons out there do not provide ANY means to detect such an "attack". Not yet anyway... SSLsniff was made to show the weakness of HTTPS connections (which not only influences webbrowsers). It is yet to be seen if browser-vendors will be able to find a solid "fix" to such vulnerabilities. Especially, since it's less of a browser issue, but more of a "connection" and "network security" problem. And when you've checked http://www.thoughtcrime.org/software/sslsniff/, you'll have seen that there's already more trouble waiting for us around the corner. This is displayed by the additional functionality the SSLsniff tool has gained... just to display related problems like the one you've recently learned about and were hoping to fix by using your browser addon.
Anyway, let me keep it short by repeat myself: Never expect a "browser addon" to provide "system protection". You may or may not like it, but that's the shortest answer to your question, wrapping it all up in a single line of understandable text. The rest of my rather long answer is merely some useful
decoration information to provide some insights where needed.