Firefox 4 is inching towards the feature freeze on 15th September. As the date nears and the pressure to finish Firefox 4 increases, Mozilla are dropping some features, so as not to delay the final release.
They have already dropped Account Manager last month. The Account Manager, as the name suggest, is supposed to take care of the user's different accounts at different websites.
Another feature they have axed is the Inspector. The Inspector is a developer tool - something like firebug. Given the number of unfixed bugs and widget related problems, they have decided to leave out Inspector to focus on the already present Web Console.
Another feature which is likely to be dropped soon is Silent Update. The Silent Update is a new update mechanism for Firefox where updates are downloaded silently in the background, like Google Chrome does. This feature has now been marked as "At Risk".