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Since I have been using Leopard on my Macbook, all the apps should be 32-bit.

So an hour ago, the macbook is upgraded to Snow Leopard. Does that mean all apps should also be upgraded to 64-bit version for speed?

For example, does Chrome, Firefox, (and any other popular ones) have 64-bit versions?

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64-bit apps have been fully supported since Leopard (OS X v10.5), and 64-bit non-GUI programs since 10.3. The only thing 10.6 added was that Apple included a 64-bit kernel (which is a completely separate thing), and included 64-bit code in (almost) all of the programs they ship with the OS.

OS X uses a "fat" binary format, which allows a single program to include multiple CPU-architecture versions of itself; when it's run, the OS picks the "best" version of the program, and runs that. For instance, on a 64-bit capable Intel Mac, it'd look for a 64-bit Intel version first; if that wasn't available, it'd look for a 32-bit Intel version; if that wasn't available, it'd look for a 32-bit PowerPC version and run that under the Rosetta emulation/translation system.

Net result: many of your programs are probably already 64-bit capable, and have been running in that mode for a while now. Some may not be, in which case they'll keep running just fine in 32-bit mode. It's all remarkably transparent.

If you want to know which of your programs support 64-bit mode, run the System Profiler utility, and select Software -> Applications from the sidebar; it'll list all the programs it can find, along with whether they're Intel-only, PowerPC only, or Universal (both), and whether they have Intel 64-bit support. You can also use Finder's Get Info on a program; if it has an "Open on 32-bit mode" option, that program includes both 32- and 64-bit versions of itself (and that option will let you override the OS's ideal of the "best" version).

Now, to answer your more direct question: I don't know about Chrome, but Firefox version 3.x comes in a 32-bit only binary; version 4 comes with both 32- and 64-bit. Either one will work fine.

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If you want to know the type of running applications, you can also use Activity Monitor, which can show the type of executable being run. –  ughoavgfhw Apr 3 '11 at 6:44
    
wow, yours is a really expert answer. does that mean the binary files that almost always "double the size" or even triple, if they include all PowerPC, Intel 32bit and 64bit versions? For example, I downloaded Xcode, and it is 4GB... 4GB, no kidding, remember when we have hard disk that was merely 20MB. So if out of those 4GB, 2.5 GB is binary, then if it includes all 3 versions, then only 800MB is really used by me? Also, why doesn't the OS "trim" the other ones if it is Snow Leopard, knowing the other 32 bit and PowerPC ones won't be used? –  動靜能量 Apr 3 '11 at 7:15
    
@動靜能量 Most of a program's size is in its resources: pictures, strings, localization, nibs/xibs, etc. An OS X program can have a fat binary with any number of architectures: OS X looks for and uses 4 archs (x86, x84-64, PPC, PPC64). As for your size question (again), if you look at a cross-platfrom app (like Photoshop or Firefox), the OS X version with usually 3 (PPC, x86, x86-64) binaries won't be all that much bigger than the Windows version with one binary. –  CajunLuke Apr 3 '11 at 14:30
    
@動靜能量 The OS doesn't trim the binaries to the local architecture so you can move them between systems with ease (so you don't have to re-download them). You, however, can remove the architectures you don't want with the lipo tool on the commandline. –  CajunLuke Apr 3 '11 at 14:34
    
@動靜能量 @CajunLuke: don't get too enthusiastic with lipo. While the OS can transparently run different programs in different modes, anything that runs as part of another program (e.g. browser plug-ins, as in @Rene Larsen's answer) has to run in the same mode as the main program. This is the main reason for the "Open on 32-bit mode" option. Also, since the kernel generally runs in 32-bit (for compatibility with older drivers), everything that loads into the kernel should have the 32-bit version left in. –  Gordon Davisson Apr 3 '11 at 15:40
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If you use FF4 then you can switch between 32- and 64-bit in the "Get Info" view of the Firefox.app.

I want to use the 64-bit (default selection) but I can't because I uses some FF add-ons that doesn't work in 64-bit - yet.

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+1 For the method to force an app to run in 32bit mode. –  mspasov Apr 3 '11 at 12:06
    
I see... in Finder, click "Applications", and when see Firefox, CTRL-click to use "Get Info", and use "Open in 32 bit mode"... –  動靜能量 Apr 3 '11 at 15:50
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