Without seeing the computer I can’t tell you exactly what’s going on, but I can give you some suggestions. In my experience Firefox itself doesn’t slow the computer down, so I’m guessing there’s something else happening and installing Firefox simply makes it more obvious. If you have less than 2GB of memory, you will probably notice slowdowns with Internet connections. You may also be infected with viruses and/or spyware. You can run Malwarebytes’ excellent scanner at http://www.malwarebytes.org to find out. Be sure you are using the latest version of Firefox, which at this time of writing is 3.6.10. You can also run a cleanup program like CCleaner, http://www.ccleaner.com, to spruce up your system before installing Firefox. Hope this helps!