VP9 encoded video content with WebM can now be played from within the HTML5 video tag on future releases of Firefox. In many ways, this is Firefox playing catch up with Google's Chrome web-browser. Chrome has offered VP9 by default for several months. The VP9 video compression standard offers many improvements over VP8.
Other changes include defaulting all plug-ins but the Adobe Flash plug-in to click-to-play in Firefox 26. What this means is that plug-ins will not be loaded automatically when websites load, but only on user request. This improves the security of the connection significantly, as websites cannot exploit old plug-in code or vulnerabilities in the last version of a plug-in anymore.
Some will have problems accessing legit sites that require plug-ins. Instead of being able to use them right away, they need to allow the sites to load plug-ins. In addition to that, you also find the plug-in indicator at the top of the page near the address of the website.