Browser WebExtension that modifies the browser behavior to switch to the left tab when the currently active tab is closed.
The approach used is possibly the fastest possible (a snapshot of the current tab and window state is continously maintained for maximum responsiveness), though unfortunately a quick flash of the default selected tab may be seen, especially in Firefox. This seems unavoidable (please open an issue if you've found a way to reduce or eliminate it).
Chrome - Chrome Web Store:
https://chrome.google.com/webstore/detail/deejpigimpefppaaafigakaegkmcbile
Firefox - Mozilla Addon Repository:
https://addons.mozilla.org/en-US/firefox/addon/activate-left-tab-on-close/