FlashGot is the free add-on for Firefox and Thunderbird, meant to handle single and massive ("all" and "selection") downloads with several external Download Managers.

* for Windows (BitComet, Download Accelerator Plus, DownloadStudio, FlashGet, Free Download Manager, Fresh Download, GetRight, GigaGet, HiDownload, iGetter, InstantGet, Internet Download Accelerator, Internet Download Manager, JDownloader, LeechGet, Mass Downloader, Net Transport, NetXfer (Net Transport 2), NetAnts, Orbit, ReGet, Retriever*, Star Downloader, Thunder (Chinese only popular product), TrueDownloader and Ukrainian Download Master [Russian language only] ), Uget, WellGet, wxDFast
* for Linux / FreeBSD / other Unix-like OSes (Aria, Axel Download Accelerator, cURL, Downloader 4 X, FatRat, GNOME Gwget, FatRat, JDownloader, KDE KGet, SteadyFlow, Uget, wxDFast ).
Many Windows-only download manager are supported on Linux through Wine!
* for Mac OS X (Folx, iGetter, JDownloader Leech, Progressive Downloader, Speed Download and wxDFast*)
* wxDownload Fast for Mac OS X is currently available in source form only

FlashGot turns every supported download manager into a download manager for Firefox!

Main good news are:

# Movable FlashGot Media button replaces status bar icon on Firefox 4 and above.
# Improved oron.com preprocessing for batch direct link downloads (similar FlashGot-supported preprocessed file sharing services are depositfiles.com, fileserve.com, filesonic.com, hotfile.com, megaupload.com, netload.in, rapidshare.com, uploading.com, uploaded.to, ul.to, zshare.com).
# Better accuracy of the Middle mouse button + drag down gesture, which triggers either FlashGot Link or FlashGot Selection, depending on the context (can be disabled by setting the flashgot.gesture about:config preference to false).
# Several performance optimizations, reducing startup time impact by a factor of 5.
# Restored Seamonkey compatibility
# Work-around for Roboform bug breaking "open link in new window" on Firefox 4 when FlashGot was installed.
# Batch download support for linksafe.me (other already supported link protection/redirection services are chooselinks.com, re-links.com, protectlinks.com, rapidsafe.net, relink.us, rsmonkey.com, hideurl.biz, ftp2share.net, rapidbolt.com, linkbank.eu, linkbucks.com, rsprotect.com
# Updated and enhanced batch download support for relink.us (other already supported link protection/redirection services are chooselinks.com, re-links.com, protectlinks.com, rapidsafe.net, rsmonkey.com, hideurl.biz, ftp2share.net, rapidbolt.com, linkbank.eu, linkbucks.com, rsprotect.com (WARNING: Feb 15 2009, the rsprotect.com web site is infected, don't open without NoScript enabled), link-protector.com, stealth.to and lix.in, urlcash.net).
# Updated Japanese translation (thanks k2jp).
# Rapidshare authorization token is "remembered" persistently by FlashGot after the first login from Firefox, in order to work around recent changes in the service forcing users to login very often.
# Many updated localizations.
# International characters in command line arguments are now supported on Firefox 4 (thanks gfdic).
# Enhanced HTTP and FTP authentication support (thanks basinilya).
# "FlashGot Tabs" hotkey relocated to Ctrl-Shift-F3 to prevent accidental reloads.
# Improved FlashGot Options|FlashGot Media panel, to choose a distinct download manager for streaming media capture. Firefox's built-in download manager is recommended because it uses browser's cache whenever possible and supports name guessing, and it's set as the default for FlashGot Media on new installatios.