Speedtest is a Firefox add-on that will allow you to frequently check if your Internet service provider is delivering the connection speed they promised.
Complete the most accurate test by clicking on "Begin Test." This appears on the small window that popups up when u click the small Speedtest button near the Address Bar.
Speedtest will automatically select the server with the lowest ping time. This means you will be connected to the closest server from a computer networking perspective.
Speedtest provides hundreds of different servers located around the globe, thereby eliminating internet congestion that negatively impacts your speed test.
The "Begin Test" method is the most reliable, but feel free to experiment with manually choosing a server by selecting the white locator dots on the "Begin Test" map.
You may also select a "Preferred Server" in your Settings to permanently give a selected server priority in any future tests. Due to Internet congestion and unpredictable ping times, you are usually better off letting Speedtest select the best server for each test.
Speedtest is cross-platform and it works on Mac OS X, Windows and Linux.