Firmware files for iPhone and all other iDevices are commonly publicly available and they will work if your device was never jailbroken before. However, unless the jailbreak guide states otherwise, make sure you use legit and official direct links to firmware upgrade files provided by reputable sources.
First of all you need to know the current firmware and baseband version of your iPhone. For that you have to go to your iPhone’s Settings menu, select General, then About. The next window will display several general details of your device, including firmware and baseband. Just look under the Version and Modern Firmware tabs to retrieve that information.
Updating your iDevice to a new firmware can be done via iTunes or via wireless. But the last one is available only for devices running iOS 5 and higher. Avoid downloading the firmware using a 3G/4G internet connection because it will significantly drain up your monthly data plan, not to mention the download will take a lot longer than via Wi-Fi. Also make sure your device is fully charged, since the download and firmware upgrade will impact battery life and interrupting the process isn’t recommended.
You can download iPhone firmware files as early as 1.0.0 and as new as 6.1.0 from the iClarified website here. Just make sure you have disabled the auto unzip feature in the Safari browser. If you’re using Internet Explorer to download iOS firmware files, change the browser’s default zip extension to .ipsw. You don’t have to unzip the file first, just rename the zip archive into .ipsw.
Once you have downloaded the desired firmware file, iTunes will give you two options: Restore or Update. Remember to use the Restore button if you’re jailbreaking, so you’ll cut the risk of something going wrong.
