This download utility is easy to use, fast, and handles a variety of files. It also works w.o. taking up a lot of memory. One of the best I've found! Edit - I've now used this add-on for over a year, and it has been a great tool for many files that I've chosen to download. It is reliable and does not cause any compatibility issues with Firefox. I would recommend it without reservation. Am enjoying it with the newest interface now as well.
Adding to the review after a year, I am still very pleased with the download app. It is reliable, efficient, and does work with many sites, but not with YouTube. I use a different downloader for that site.