$ grunt firefox
build/firefox/shumway.xpi shall be created.
Open (or drag'n'drop) and install the shumway.xpi file in the Firefox Desktop or Firefox for Android.
Open the browser on the page with flash movies. You shall see the Shumway extension interface (if the Flash plugin is installed, there will be a "Shumway" fallback button as shown on figure).
For Firefox 21 and up, the extension is shown only when the click-to-play is enabled (and native Flash plug-in is installed). There is a way to bypass this check and always show the Shumway extension instead of the plug-in: open "about:config" and add a new boolean preference "shumway.ignoreCTP" with value "true" (restart of the extension or Firefox is required). If click-to-play is enabled, for Firefox 23, set "Shockwave Flash" to "Ask to Activate" on about:addons plugins tab.
See also Debugging and Configuring Shumway.