I don't know about the XecureBrowser itself, but secure browser operating systems is a huge research area at present. SSL *has* been cracked --- there're a couple of known timing and man-in-the-middle attacks --- but if you use good keys, and disable attacking sites (they'd be pretty obvious: see the paper http://crypto.stanford.edu/~dabo/papers/ssl-timing.pdf for the classic explanation of one such attack -- there are a couple more) you should be safe for now.
For secure browsers, see the recent paper on the Illinois browser operating system: http://www.usenix.org/events/osdi10/tech/full_papers/Tang.pdf Peter C -- SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/ Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
