For a long time now I have been aware of this feature but still I think many users of Internet Explorer do not know about it. The feature that I'm talking about allows you to save a complete web page INCLUDING pictures to a single file so it can be used later without having to connect to the internet. This feature is accessible from the File->Save As... menu after you select the Web Archive (mht file) as a type of the saved file:
Note that there is also another option to save an entire page using the same dialog box and selecting Webpage, Complete option. This however creates one file for the page and a folder containing the rest of the required files such as images. This is not a very ellegant solution in my opinion.
As good as this feature is, there are occasionally problems with saving the page and the only information is that there was some kind of error (don't we love this kind of messages?). Other than that I think you will love it 
BTW: I haven't seen such a feature in the standard installation of the FireFox but I suppose that there is some plugin just for that.