HTML5 is the proposed next standard for HTML 4.01, XHTML 1.0 and DOM Level 2 HTML.
It aims to reduce the need for proprietary plug-in-based rich internet application (RIA) technologies such as Adobe Flash, Micro$oft Silverlight, Apache Pivot, and Sun JavaFX.
HTML5 introduces a number of new elements and attributes that reflect typical usage on modern Web sites. Some of them are semantic replacements for common uses of generic block (<div>) and inline (<span>) elements, for example <nav> (website navigation block) and <footer>. Other elements provide new functionality through a standardized interface, such as the <audio> and <video>[5] elements.[6]
Some deprecated elements from HTML 4.01 have been dropped, including purely presentational elements such as <font> and <center>, whose effects are achieved using Cascading Style Sheets. There is also a renewed emphasis on the importance of DOM scripting in Web behavior.
The HTML5 syntax is no longer based on SGML despite the similarity of its markup. It has, however, been designed to be backward compatible with common parsing of older versions of HTML. It comes with a new introductory line that looks like an SGML document type declaration, <!DOCTYPE html>, which enables standards-compliant rendering in all browsers that use "DOCTYPE sniffing".
I have wrote and built webpages using HTML 4.01 codes, I guess now I will have to relearn all the new codes
In other word's the new HTML codes will have the plugins codes already in the markup.
What version of firefox do you have? I believe the update of HTML5 is version 3.6.
http://www.mozilla.com/en-US/firefox/ie.html