Over the last few days, Mozilla's lovely Are We Pretty Yet? (AWPY) has been steadily accumulating Firefox 5 interface mock-ups. If you haven't visited AWPY before, its primary purpose is for listing and fixing bugs in the Firefox 4 UI -- but it seems it will now also be used to showcase potential interfaces for Firefox 5.
Firefox 5 UI Mock-ups
The mock-ups include concepts for 'desktop apps', which are Firefox's answer to IE9's pinned sites, and hopefully they'll re-use the same meta tags so that website developers won't have to accommodate both browsers. Presumably a similar interface will be used for Open Web Apps when they finally emerge, too.
Firefox Sync looks like it will be more prominently advertised, both on the about:home default start page, and in Remember Password and Edit Bookmark dialogs.
Finally, it looks like enabling/disabling and updating add-ons could be significantly streamlined in Firefox 5. It seems a little odd to break out into a separate window after moving to a tabbed add-on manager in Firefox 4, though.