ScrapBook
Open Source
ScrapBook is an extension for the Mozilla Firefox web browser which adds enhanced scrapbooking, page saving, bookmarking, and notetaking functionality.
SiteSucker
Commercial
SiteSucker helps you download websites to your computer.
UnMHT
Open Source
UnMHT adds MHT file reading/writing support to Firefox and SeaMonkey.
Compare Offline Explorer with alternatives
Compare ScrapBook and Offline Explorer and decide which is most suitable for you.
Compare UnMHT and Offline Explorer and decide which is most suitable for you.
Compare SiteSucker and Offline Explorer and decide which is most suitable for you.