Mozilla has begun showing UI design mockups for Firefox 5, with deeper cloud integration being a primary design interest.
Larger software development companies like Mozilla have roadmaps that extend out years, and many times the next major version of a product is in development well before consumers get the finalized previous version. Such is the case with Firefox, and today we have a glimpse of what features might be coming in the next version.
As reported by ZDNet (via a ConceivablyTech article), Firefox 5 will bring a host of improvements to the general interface. One of these is the concept of menus being specific to the site you are on. It is unclear how customizable this feature will be, but in the UI mockups we see Twitter as a candidate for such a menu.
An example of dynamic menus generated based on the website you are viewing.
Remember though that these are just conceptual mockups, and there is no guarantee any of the features will make it into the final version we get. However if Mozilla's current technology previews are any indication, Mozilla is planning on further blurring the line between cloud and desktop applications.