WebNarrator is available for Chrome and Firefox. Pick your browser and install from the official extension store.
Chrome
Firefox
After installing
Once the extension is installed, select any text on any webpage and press Option+Shift+S (Mac) or Alt+Shift+S (Windows). WebNarrator will read it aloud.
You can also right-click selected text and choose Read selected text aloud, or click the WebNarrator toolbar icon to open the popup.