Download Manager (S3) by Oleksandr

What is Download Manager (S3)?
Despite its compact size, Download Manager (S3) packs in a bunch of useful features that standard download utilities don’t provide. Its fully customizable interface automatically disappears when not in use - giving you full control without interruption.

Features:

• Displays download progress, time remaining, full file path, URL, etc.
• Hides status bar when all downloads are complete
• Position buttons in the download bar where you prefer
• Ability to show only the number of completed downloads
• Preview image files
• Can pause, cancel, resume, or re-try single and group downloads
• Replace Downloads Library page - you have full control of your download history
• Change the text and tooltip sizes
• Has a Queue mode
• Shows «Download complete» notification

Hotkeys (by default for Windows):

• Ctrl + J = open Download History page
• Ctrl + Shift + Z = show/hide Download Bar
• Ctrl + Shift + Backspace = clear the last downloaded item
• Ctrl + Shift + Insert = undo last clear
• Ctrl + Shift + Enter = open the last downloaded item
• Alt + Shift + Enter = show the last downloaded item

WebExtensions Limitations:

• Download Bar not displayed on some sites from mozilla.org and google.com
• Download Bar not displayed on sites that do not have pages, for example error 404 (page not found)
• Download Bar not displayed on internal Firefox and GoogleChrome pages, for example about:addons , about:config , chrome://extensions/ , chrome://settings/ , etc.
• Download Bar not displayed on the extensions pages "moz-extension://...." , "chrome-extension://...."

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You also can use Download Manager (S3) for Google Chrome:
https://chrome.google.com/webstore/detail/download-manager-s3/ngmcjenopiialholjggicgplegmcmjda
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Download Manager (S3) is full compatibility for Google Chrome, but there are known problems for Firefox:

• downloadItem.exists does not change from true to false when a file is removed
  https://bugzilla.mozilla.org/show_bug.cgi?id=1381031
• does not display history from past sessions
  https://bugzilla.mozilla.org/show_bug.cgi?id=1255507
• chrome.downloads.drag is not a function
  https://bugzilla.mozilla.org/show_bug.cgi?id=1245648

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forums mozillaZine:
http://forums.mozillazine.org/viewtopic.php?t=2793995

forums mozilla-russia:
https://forum.mozilla-russia.org/viewtopic.php?id=73317