You may have ad / mal-ware. Further information can be found in the Troubleshoot Firefox issues caused by malware article.

Run most or all of the listed malware scanners. Each works differently. If one program misses something, another may pick it up.

Not adware/malware. MSN take over when Firefox is opened. HG

They put a program on your system that changes your settings. It's the same problem.

Open your Programs Folder. Look for anything that you don't know.