With Firefox 57 a lot of legacy extensions stopped working (see the list Discontinued Legacy Addons for Firefox ). Luckily some developers created new WebExtensions as replacement of these popular addons, trying to restore all their functionalities in spite of WebExtensions API limitations. This a list about the most interesting ones I have found.
Foxy Gestures implements mouse gestures for Firefox and is heavily inspired by 
FireGestures .
Foxy Gestures implements mouse gestures for Firefox and is inspired by
Chrome Download Manager. Bring Google Chrome like Download shelf into Firefox.
You can track download progress, open containing folder by default and manage all downloads like in Google Chrome provides.
This extension reproduces the
Google Chrome Download bar in Firefox and can be used as alternative to
Download Status Bar and
Download Manager (S3) . Since WebExtensions APIs don't allow the creation of new bars, the extension injects HTML code in your page to visually emulate a bottom statusbar with your current and recent downloads.
This is an experimental Firefox 57 download manager with multithreading support.
If you are missing
DownThemAll for its multithreading support and you can't wait until its new release, this extension will do the trick.
Copy PlainText is a tiny multi-process extension that removes the formatting from the selection before saving to the Clipboard.
Copy PlainText, as suggested by the name, is a replacement for and . It is Multi Process compatible and offers 2 keyboard shortcuts: F7 and the original Ctrl+Shift+F7. Anyway user can't define his own keyboard shorts and Copy PlainText can't be set as the default copy function of the browser.
Get rid of a site's cookies as soon as you close its tabs. Protects against trackers and zombie-cookies. Trustworthy services can be whitelisted.
Self Destroying Cookies is a WebExtension clone of
Self-Destructing Cookies : it automatically removes cookies, after the tab closes or you leave the website, but it's not yet able to clear the LocalStorage, due to API limited access.
MySessions is a simple session manager. You can quickly save your current browser state and reload it whenever necessary.
MySessions is a simple session manager and crash recovery addon. It was made as an alternative to
Session Manager and can import saved sessions from it too.
Conex is a replacement for the discontinued
Tab Groups : it implements groups via containers and integrates a search function for tabs, bookmarks and browser history. At the moment hiding and showing tabs (one of the core features) is blocked by API limitations.
ReloadMatic automates the periodic reloading of browser tabs. It is the spiritual successor of the 
ReloadEvery add-on but with additional features and updated for new browser versions.
ReloadMatic is the so-called spiritual successor of
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