UnMHT is a add-in for the Firefox browser which will allow you to save multiple open tabs as separate .mht files. .mht files are web archives files which are a combination of html codes and associated files which are usually links to external files in html code. So as you can open multiple web pages referenced as links in a batch file (see the Tip of the Night for July 29, 2016) and then save these web pages using this add-in.
The add-in appears as a small brown round icon (shaped like the firefox one) on the toolbar.
If you have multiple tabs open you'll see that you have option to save each of them separately. You have the option to name the mht files with the web page titles, which can be very helpful when you're indexing.