The Web languages
JavaScript,
Java, ActiveX etc. are designed to be secure so
that a Web applet cannot access private data on your computer. However,
sometimes a bug creeps in. In Netscape Navigator HTML, it is possible to
send the contents of a form by email, and also to upload a local file to the server.
In JavaScript, it is possible to perform some action when a page loads, or
when the mouse is over a particular object. Certain versions of Navigator allowed a
malicious Web author to combine these functions, secretly uploading a password file
or mailing some data with the user's email address to the server without the user's
knowledge.
See www.digicrime.com/exploits for examples of this.
Also Cache-Cow
It is unwise to blindly upgrade browsers to the latest version, particularly a major revision such as X.0, without a good reason. During the life cycle of a piece of software version X.0 introduces several new features, some of which may not work properly. Version X.1 fixes some of the problems but may introduce some more bugs. Version X.2 fixes a few more bugs, and so on. Be patient; wait a bit. Read the release notes. Read the security advisories. By all means try a beta release for development; just don't expect it to be as secure and stable as a mature version.
A.Daviel