ContextSearch web-ext by Mike B

If you are experiencing issues, check https://github.com/ssborbis/ContextSearch-web-ext/releases for signed pre-releases with the latest fixes

ContextSearch web-ext

The most powerful, most complete search enhancement for Firefox

This Add-on was written as a Firefox 57+ web-extensions compatible replacement for Ben Basson's popular Context Search Add-on but has grown into much more.

• Import your currently installed search engines
• Access search engines from the Context Menu
• Manage search engines using subfolders and separators
• Access search engines from a popup menu (QuickMenu)
• Compatible with all search engines using POST or GET requests
• International character-set support
• Reverse-image search
• Site search
• Add new search engines simply with a right-click
• Custom API to build and install custom OpenSearch engines to FIrefox Search Toolbar
• Edit search engine parameters and icons
• Access bookmarklets from the context menu and quick menu
• Highlight search terms
• Lite and Dark themes
• Use custom CSS for menus

* Linux and MacOS X users may need to set the option ui.context_menus.after_mouseup to true in about:config for mouse actions to work properly

Permissions
<all_urls>
This addon is works on all websites
activeTab
This addon injects code into tabs to create menus and styling
contextMenus
This addon provides a context menu
search
This addon interacts with Firefox search engines
storage
This addon saves preferences
tabs
This addon provides means to open search results in new tabs and windows

More info @ https://github.com/ssborbis/ContextSearch-web-ext