Mozilla developers added the support for viewing the files downloaded and stored in the cache by simply typing in about:cache in the Firefox address bar. This shows you all the cache storage located on the disk in form of files as well as the cache loaded in the Firefox memory at the moment. Also the offline app cache can be viewed through this method. But it does not show you any preview of the files stored in the cache, you basically have to reopen these files in the web browser all over again in order to see the previews.
You can make things a little easier using the CacheViewer extension for Firefox. When using this extension, you can not only view a list of all the entries stored in the Firefox cache, but you can also have a preview of the media files like the images and videos. It includes all types of cache entries – both the ones located on the disk as well as the ones loaded in the memory. At the bottom of the CacheViewer window you can see how many entries are located in the memory or on the disk. You can also see the cache storage space used by these entries.
In the CacheViewer window, you can select any entry to see its preview in the tiny frame. You can have a preview of only media files like PNG images, JPEG images etc. I tried selecting some of the GIF images, but could not see any preview for them. But good thing, and I think the most useful feature for this extension, is that you can right-click on any file and choose to save it to anywhere on your computer. You can also copy any cache entry’s URL location. This way you can easily locate and save the video files or other large files that you have already downloaded and that are already in the cache without having to reopen them through the internet.
In the settings for the CacheViewer there is nothing much except an option to decide whether it should be opened in a new window (this is the default) or if you want it to be opened in a new tab.
Conclusion: CacheViewer makes it much easier to view and save the entries stored in the Firefox cache. You can also have a preview of some of the media files stored in the cache.
You can get the CacheViewer extension for Firefox from https://addons.mozilla.org/en-US/firefox/addon/cacheviewer/.