Since 2009 onwards Google is working on SPDY (pronounced as “speedy”), an experimental protocol for the faster web and and alternate for HTTP which aims to reduce the latency of pages.
Google uses SPDY feature for most of its services like search, Gmail, YouTube etc. and the Search engine giant has added this protocol to Chromium and Chrome browsers. Recently Mozilla joined with Google on this project to avail faster load of pages to its users.
UPDATE: Google has removed support for SPDY from Chrome in version 51 and has moved to HTTP/2. Below extension for Chrome and Firefox lets you know whether a site is HTTp/2 ena supports HTTP/2 or not.
And the Extensions are:
A Green Indicator for SPDY Supported Site in Address bar of Firefox and Chrome After installing SPDY Indicator Extension
It is very difficult to know which sites serves via SPDY in Firefox browser. Developer Guillermo Rauch created Chrome SPDY Indicator extension which shows green icon in the address bar for loaded SPDY site, other wise shows grey icon for non-SPDY sites.
There aren’t much SPDY enabled sites right now so you most likely to see grey icon in the address bar unless you visit Google, Gmail,Twitter, YouTube and other Google related sites in Chrome, you can disable this grey icon in address bar from the extension options.
You can install this extension from from Chrome web store, here.
Inspired from this, another developer Cheng Sun created SPDY Indicator for Firefox which you can download from AMO.
You can try Twitter, YouTube, Gmail, Google.com and other SPDY supported sites you know in Firefox and Chrome browsers after installing this extension.