Firefox 4 is an i386/x86_64 universal binary. The default architecture on Mac OS X 10.6 and above is 64-bit. The default architecture on Mac OS X 10.5 is i386. See system requirements for details on supported systems.

For details on how to install, see Installing Firefox on Mac.

Firefox 3.6 will work on 64-bit versions of Mac OS X, but it is not a 64-bit application. See system requirements for details on supported systems.

Firefox 4 will run in 64-bit mode. For details on how to update Firefox, see Updating Firefox.

There is a x86_64 binary of Firefox 4 to run on 64-bit versions of Linux. See system requirements for details on supported systems.

For details on how to install, see Installing Firefox on Linux.

Firefox 3.6 will work on 64-bit versions of Linux, but it is not a 64-bit application. See system requirements for details on supported systems.

Firefox 4 will run in 64-bit mode. For details on how to update Firefox, see Updating Firefox.