The new version of Firefox include mostly incremental changes to the current version curtsey of the recent 6 weeks update cycle. Amongst those changes /improvements and fixes you can find (full list):

Firefox started losing users to Google's Chrome around 2010 and by late 2011 Chrome has already surpassed Firefox and will soon pass the 30% global browser user mark. Internet Explorer still dominates the market with between 32% and 35% of global browser users however it is quite clear that for the first time ever Microsoft is going to loose the browser crown and Google will become the new global browser leader. This change might force many of the older website owners, who's site do not support the latest standards, to conform to the change in the market.
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You will soon be able to download the new Firefox 11 (free as always) from the Mozilla website (direct download link).
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TFOT covered many browser related stories in the past starting from the launch of Firefox 3, which included more than 15,000 updates compared to its previous version (just "slightly" more than the current version), as well as major restructuring to provide improved performance and stability, delivering, Aurora, Mozilla’s future browser and  Cool on ”Knol,” from Google which allows users to contribute their knowledge in a wiki-like format. We also covered the initial release of Chrome in 2008 (currently on its 17'th version already).