If you are looking to test the actual speed of your Internet connection, the traditional way is to use a standard Internet speed tester, such as the Speakeasy Speed Test. An alternate way Firefox users might want to explore is the Bandwidth Tester extension.

The difference between the two is explained rather well on the Bandwidth Tester’s page:

This tool is more accurate than many of the website testers because it downloads several files at different sizes to find your average connection speed. However, this is also dependant on where you are in relation to where the test files are being served from. You can change the test files used in the Bandwidth Tester options.

To me, the explanation makes a lot of sense as when you are browsing the Internet you are typically downloading many smaller files instead of a single big file. This is a good way to make sure you are utilizing the Internet speed you are paying for.