Violentmonkey provides userscripts support for browsers. It works on browsers with WebExtensions support.
Example Firefox add-ons created using the WebExtensions API
Firefox/Chrome browser extension. SmartProxy will automatically enable/disable proxy for the sites you visit, based on customizable patterns.
The DownThemAll! WebExtension
Tree Style Tab, Show tabs like a tree.
A command line tool to help build, run, and test web extensions
An awesome browser extension that reads aloud webpage content with one click
Firefox extension to save and restore tabs as bookmarks. Clear your tabs, clear your mind.
A browser extension to copy tabs and links as Markdown
Charter and administrivia for the WebExtensions Community Group (WECG)
Some web pages use intermediary pages before redirecting to a final page. This add-on tries to extract the final url from the intermediary url and goes there straight away if successful.