Without care, users may have to login twice to gain full access. There may be additional changes for a server with SSL capabilities and there are other methods also being tried. SSL is the most though. Any time a form is processed, programs run on the host computer on behalf of the client. This has actually happened.
Few developers know enough about security to keep out determined hackers.
Best to have a consultant who specializes in security evaluate the site before putting it on line. Make your programmer guarantee a reasonable level of security before contracting with them for the project. I would say that no matter how tempting it is to include features in your web site that cater to a particular browser it is generally a bad idea to do this. Netscape and MS Internet Explorer would love nothing more than for designers and programmers to incorporate lots of their special extensions in new pages. This would help to insure that people purchase and use those browsers.