One of the most useful functionality in Firefox is a button to subscribe to RSS, if the website we visits has a RSS feed. The button is always in the right-most of address bar. But, since Firefox 4.0 the button is missing from the address bar. I though this functionality is deprecated, but luckily it’s not. The button is shown on the toolbar by default anymore.
In order to show the RSS Subscribe button you have to click on Firefox menu -> Preferences -> Toolbar Layout. From here a Customize Toolbar window will be shown, like shown below. Find the RSS Subscribe button and then drag the button to the toolbar. You can choose the place in the toolbar as you like. For example, I put the button at the right side of the address bar, just besides Home button.