Firefox 5 coming in June

Now, I must admit that I am a bit confused.  It took Mozilla over two years to get us from version 3 to 4, and now they are moving to version 5 in 2 months.  Mozilla has announced that Firefox 5 will be coming out June 21, 2011.  They intend to hold to a more frequent release schedule, preferably about every 6 months.  This is similar to Google Chrome.