If you’re using OS X 10.5.8 Leopard then you can still download Firefox 16 for Macs which works on 10.5.8. However, we strongly recommend you upgrade to at least OS X 10.6 Snow Leopard or higher as Firefox 16 is no longer updated by Mozilla and therefore not secure anymore. Note that this only works if you’re using an Intel Mac – if you’re still using OS X 10.5 or 10.4 on a PowerPC Mac, then we recommend this article about how to install Firefox on PPC Macs and OS X 10.4.11. In this case you’ll need Firefox 3.6 for Mac which was the last one to support PPC Macs but is even older and more vulnerable than Firefox 16. In this article, we look at the risks and problems of using Firefox 16 and highlight what your options are.
WARNING: Firefox For 10.5.8 Is Not Safe!
Since Firefox 17 until the latest version of Firefox, you need at least OS X 10.6 Snow Leopard or higher to run the browser safely and efficiently on your Mac. We emphasize that using Firefox 16 is not a good idea because it is no longer updated or supported by Mozilla leaving it vulnerable to attack, malware, spyware and other online threats. Not only this, but you’ll also find that many add-ons or plugins simply won’t work with it anymore. If you want to see just how many security threats have to be fixed on a regular basis by Mozilla, check out the latest Security Advisories for Firefox. Mozilla take security very seriously and it’s also worth checking out the Mozilla Security Blog to see some of the work that goes into keeping Firefox safe.
Why You Should Upgrade Your Mac
An even more compelling reason to upgrade from OS X 10.5 is that since 2009, Apple has stopped supporting Leopard anymore meaning the entire OS is vulnerable to attack and serious stability issues. It’s also highly likely that Apple will also stop supporting OS X 10.6 Snow Leopard soon too, so our advice is try to upgrade to at least OS X 10.7 Lion. To do this though, your Mac will need to be an Intel Mac with a minimum of Core 2 Duo, i3, i5, i7 or Xeon processor and at least 2 GB of RAM. If you’re unsure about your Mac’s specification, go to the Apple icon in the top left corner of our screen and select “About This Mac”. 
Officially, you’re also supposed to be running at least OS X 10.6.6 in order to upgrade via the Mac App Store (which OS X 10.5 can’t access) but there is a way to install Lion from a bootable DVD or USB drive. If you do decide to upgrade your OS X or Mac, you don’t have to lose all of your Firefox settings and bookmarks. You can use Apple’s Migration Assistant to transfer everything from your old Mac or OS X, including Firefox.
If you have any other questions about using Firefox for OS X 10.5.8, let us know in the comments below.
running osx 10.5.8 2 ghz powerPC G5. Downloaded Firefox 16.2 and the application folder says it can’t open because firefox not supported by this architecture. Help
William, Unfortunately Firefox 16 doesn’t work on PPC Macs. The last Firefox version to work on PPC Macs was Firefox 3.6.28 but it only works up to OS X 10.4.11. Alternatively, there’s also a project called TenFourFox which might help you. We’ve explained how to install Firefox on PPC Macs here.
Clicking on all the how to install firefox on PPC Mac or 10.5.8 is circular, and does not tell me how to do it. How do I get firefox on a G4 with 10.5.8?
Firefox no longer supports PowerPC Macs like the G4. But you can still download the last version that worked on PPC Macs although we don’t recommend it as its very insecure and doesn’t work well. This article on how to install Firefox on PPC Macs should help you.
Model Name: MacBook
Model Identifier: MacBook1,1
Processor Name: Intel Core Duo
Processor Speed: 1.83 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 2 MB
Memory: 2 GB
Bus Speed: 667 MHz
Boot ROM Version: MB11.0061.B03
SMC Version (system): 1.4f12
Serial Number (system): 4H6308J3U9B
Hardware UUID: 00000000-0000-1000-8000-0016CBCE0A55
Sudden Motion Sensor:
State: Enabled
PLS TELL ME WITCH KIND OF WINDOW I CAN USE IN THIS PC