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 inspired by 
FireGestures .
Unfortunately, there are many limitations due to limited or absent Web Extension APIs: extension cannot integrate into about:newtab, about:home, view-source:, etc. Nor will it work on addons.mozilla.com
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.
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, as suggested by the name, is a replacement for 
Copy Plain Text and 
Copy Plain Text 2 . 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.
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 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 is the so-called spiritual successor of 
ReloadEvery with some additional features like reloading the page by skipping the browser's local cache, by random intervals or only if there are no network or server-side errors.
