The announcement on the Mozilla blog promises:
Firefox manages memory more efficiently to deliver a nimble Web browsing experience. Users will notice Firefox is faster at opening new tabs, clicking on menu items and buttons on websites. Heavy Internet users will enjoy enhanced performance when lots of tabs are open and during long Web browsing sessions that last hours or even days.
They have also added an option to send performance data to Mozilla to help with future improvements, which you will be asked about the first time you run Firefox 7.
Other changes include modifications to Firefox Sync intended to make passwords and bookmarks update almost immediately and a new rendering engine which should speed up HTML5 2D animation rendering for most Windows 7 and Vista users.