This will show you an easy way to save files from the Firefox temporary cache, and rename them with proper extensions.
First, decide what folder you want to save your files to, let's say Documents. Open Documents, right click on an empty part, choose 'Create New', then 'Link to Location'. When the dialog open browse to ./.mozilla/firefox/[your folder]/Cache/ and give your link a fancy name like Cache. After you've made it you can click on Properties for it, then click on the icon on the properties dialog and browse the Icon section for another icon, there is even one for Cache. Click your link to made sure that the cache folder actually opens when you use it.
Next, clear your cache in the Firefox tools menu, then go to the site where you want to get files and go through whatever process is involved, such as viewing pictures, browsing web pages, etc.. When this is completed make a new folder for your new files in Documents. Then open the Cache folder, choose Edit, Selection, Select All, then Edit, Copy, then paste those files into your new folder,
Your next task will be to purge and sort the files. Firefox give the files in Cache an arbitrary hexadecimal name with the last three digits d01. You will have to sort them by their appearance in the folder window. Let's say you're only interested in files of the JPEG type. One way or another remove all the files not of that type, either by deleting them or moving them elsewhere.
Now the third and final step, giving the files a proper file extension. To do that just open a terminal in your folder by right clicking and choosing Actions. Then enter this code into the termiinal.
for FILE in * ;do mv $FILE "$FILE.jpg";done
You can change .jpg to .html, or whatever.
And that's all there is to it.