If you download the latest version of Firefox today, you'll see much more than the odd bug fix or functionality improvement - the UI has been completely redesigned.
According to this post on the subject, the UX team used "the 80/20/2 rule" to reprioritise the browser's features according to how much they are used.
They used heat map studies to discover which features were used the most (the 80 per cent use case) and placed them most prominently. Features used from time to time (the 20 per cent use case) were placed inside a menu, and rarely-used features (the 2 per cent use case) can be added by the user with the new customisation interface.
The customisation page enables people to choose whether functions appear in the toolbar or the menu, and to add functionality beyond what is shipped by default.
The overall look has also been redesigned. Tabs are now smooth and curved, and the active tab is emphasised while the others recede into the background.
The release notes are here. Highlights for developers include the implementation of box-sizing and a handy colour picker widget. You can find out more in this video:
Mozilla has also launched The Web We Want campaign, which asks people to share what they want the web to be able to do for people, whether that's promoting freedom, safeguarding privacy, creating opportunity or something else. This data will be made available over the coming weeks.