When you install Firefox with Windows 7, you should be able to just bring it up and use it. That's always worked for me on the 5 PCs I have at home and the 28 I support. Did you get any errors when you installed it? Do you get any error messages when you try to use it? If so, please post the exact text of these. Does Internet Explorer work? Have you tried Chrome? You can download Chrome from http://www.google.com/chrome . Two other possibilities are viruses and corrupt system files. Get your antivirus program up to date and boot into Safe Mode. Note that some viruses can hide themselves from your antivirus program in normal mode, so you really need to scan in Safe Mode. To get into Safe Mode when you first power up, hit F8 about once a second until you get the menu and select Safe Mode. Then run a full system scan.
You can check for corrupted system files. Open an administrator command prompt and run SFC if the above doesn't help. Click START, then type CMD in the search box, right-click CMD.EXE and click Run as administrator. Then from the command prompt type sfc /scannow.
I bring it up but it will notwork. I have windows 7.