It's quite simple. I downloaded Firefox 4.0, quit the existing application, overwrote it with 4.0 and tried to reopen Firefox, whereupon I received an error message saying that 4.0 was not compatible with this computer (which runs OS 10.4.11). So I had to go onto Safari, search umpteen sites -- since Firefox has managed to replace most free downloads of earlier versions with ones of 4.0 -- and find Firefox 3.6.16, download *that* and overwrite 4.0. Now I'm back in business but no thanks to version 4.0 ... despite its (bogus) claims of OS 10 compatibility. Perhaps that should be "Compatible with certain unspecified sub-versions of OS 10." That'd be a lot closer to the truth, in my opinion.