>>17263641firefox 4 just came out. It's purpose was to modernize the UI and browsing features. Also to kill hundreds of bugs not squashed since Firefox 2, as in over 600. That;s why it took a year and a half to finish.
Now that these bugs are gone, Firefox went to rolling releases, and there have been another 200 bugs killed releated to new features in 5 and 6. Things just keep getting complicated. Whatever features that are ready for release a year after 4 will appear in 5. two years in 6, etc.