Floating elements were breaking the design and we needed to set a fixed width or use hacks like faux columns for the layout to work or use JavaScript to set the height of the element.
As the sticky navigations are quicker to navigate and are always available no matter what part of the page is being shown to the user, developers have adopted this approach in a lot of websites.
The following browsers support this feature: Opera 9.5+, Chrome, Firefox 3.5+, and Safari 1.1+. Historically, this feature was only supported by Safari, but now all recent major browsers support this property except IE.