Since 2009, Google has been working on SPDY (pronounced “speedy”), an experimental protocol for the faster web and alternative to HTTP that aims to reduce page latency.
Google uses SPDY feature for most of its services like search, Gmail, YouTube etc. and the search engine giant added this protocol to Chromium and Chrome browsers. Recently, Mozilla teamed up with Google in this project to achieve faster page loading for its users.
Mozilla has added SPDY support for Firefox which is disabled by default and you see the network.http.spdy.enabled preference value set to true in about:config.
TO UPDATE: Google removed SPDY support from Chrome in version 51 and switched to HTTP/2. The extension below for Chrome and Firefox lets you know if a website is HTTp/2 and doesn’t support HTTP/2 or not.
And the extensions are:
A green bookmark for SPDY supported website in Firefox and Chrome address bar after installing SPDY bookmark extension
It is very difficult to know which sites serve via SPDY in the Firefox browser. Developer Guillermo Rauch created Chrome SPDY Indicator extension which shows green icon in address bar for loaded SPDY website, otherwise shows gray icon for non-SPDY websites.
There aren’t many SPDY-enabled sites right now, so you’re more likely to see the gray icon in the address bar, unless you visit Google, Gmail, Twitter, YouTube, and other Google-related sites in Chrome, you can turn this off. gray icon in the address bar of the extension options.
You can install this extension from the Chrome Web Store, here.
Inspired by this, another developer Cheng Sun created SPDY indicator for Firefox that you can download from AMO.
You can try Twitter, YouTube, Gmail, Google.com and other SPDY compatible sites you know in Firefox and Chrome browsers after installing this extension.