One of the first browsers was called "Mosaic" (it was very important, but could do nothing more than display very basic HTML!). And "Mozilla" was the "Mosaic Killer" (it's in the name; it could do a lot more than Mosaic). So every browser, which wants to be better than Mosaic, will state so in it's user agent. Today, every browser out there is better than Mosaic, so everyone will state that they are Mosaic killers and can do more than basic display functionalities.