well, for 1, microsoft's IE is not free, its a bundle with the OS. which alot of people are arguing about its value and dominance, as well as monopoly in the browser market.
as for google's chrome, imagine them one of the biggest, if not the biggest, search engine now, what is the next best logical step to go? firefox already implemented search direct to google on the browser, google chrome just make it even better.
then back to firefox... without mozilla, opensource stuffs, i think all of us will be paying a fortune for every software available... its an alternative. same like you don't have to use Windows, and if you want it free, you can use linux OS, like ubuntu, etc. if you want free spreadsheet, wordprocessor, you can use OpenOffice... these are free alternatives relying on donations and sponsorships. and like anything free, don't expect miracles.