Do you open large number of tabs in the background, so many that the page you are viewing starts to crawl because your Internet connection is unable keep up with simultaneously loading web pages? Unless you live in Japan with insane connection speeds, this is a common problem with folks around the rest of the world, particularly in a place like India. What you need is theLoad Tabs Progressively addon for Firefox.
As obvious from the name, Load Tabs Progressively loads background tabs one after the other. You can limit the number of tabs Firefox can open concurrently so that your connection is not overwhelmed with requests and data from downloading pages.
The default value is 3 which means tabs will be loaded 3 at a time, which should be quite comfortable but you can pump up that figure if your net connection can take more. Alternatively, you can reduce the value if you are on a slow connection or if you are downloading something else through another program like a Torrent file.