HTML5 is the probable replacement of HTML (Hypertext Markup Language), which is the core markup language of the internet. However, HTML5 is still in its initial stage of development and therefore, some websites do not work when HTML5 support is enabled on a browser such as Firefox 4. A common example includes the issues that users are facing with Windows Live Mail due to Firefox Four’s HTML 5 support. HTML5 Toggle is a Firefox add-on that enabling or disabling HTML5 support in a single click.
After installing the extension, HTML5 support can be enabled or disabled from Tools –> Disable HTML 5 or by adding the HTML5 Toggle button to a toolbar. Unfortunately, this button is not added to any of the browser toolbars by default. However, it can be added by going to View –> Toolbars –> Customize, and dragging the button to a preferred location on the browser (e.g. next to the address bar). This allows enabling or disabling HTML5 support by simply clicking on this button.
Get HTML5 Toggle for Firefox