According to the Mozilla Wiki, future versions of Firefox will use separate processes for the browser user interface and the handling of web page content. Unlike Google Chrome and Internet Explorer 8, the individual browser tabs will not run their own individual processes – this is only planned for even later versions. The Mozilla developers say the advantages will be improvements in performance and stability. By running the UI and content in separate processes, if there are problems with a web site the browser won't lock up and will still react to user input. Current versions of Firefox only run as a single process.
A semi-functioning draft of the browser is planned for mid July, followed by fleshing out the main code by the start of November and then final compatibility and performance tweaks. There is as yet no estimated date for the final release.
Mozilla is also considering using the network stack from Chromium to replace Necko, its own network library.