Apple offers firmware files (IPSW) to the developers so they can install it on their registered devices and test their applications. However, the IPSW files have been much more useful to upgraders/downgraders who jump between various iOS versions on their iPhone/iPad/iPod.
It's said that with iTunes 11.1, Apple is closing down on custom restore (which means you cannot use a custom IPSW file / Shift+Restore) but even if that comes to pass, there will be other methods to push a custom IPSW file downloaded from the web to your iDevice. But where do you get the firmware files for iPhone, iPad, iPod Touch and Apple TV?
Turns out, there are tons of resources for this very thing. Here's some of the trusted places where we get our IPSW files from:
IPSWDownload /IPSWDownloader is probably the only source you need to download IPSW files for your iDevice. You have to select the device (ex., iPhone), the model (ex., 5s) and the firmware version (ex., 7.0). Click on Download and the IPSW file should start downloading automatically.
has a large list of iPhone firmware files for all iOS versions including the latest as of writing this, 7.0.2. In fact, several other tech websites like RedmondPie, iDownloadBlog and OSXDaily provide direct download links for all the IPSW files.
More Resources to Download Firmware (IPSW) Files for iPhone, iPad, and iPod Touch:
It's better to prefer IPSWDownload.com because you don't get lost in the plethora of download links.
Remember, IPSW files are large. Many of them are about 1GB large and may take time to download in-browser. In Windows, the files may be renamed upon download. Browsers typically rename IPSW file format to a zip format but you can change it back so iTunes recognizes it when you use it.