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.

Does it look anything like this one, which mentions a particular add-on:

You can review and disable, and often remove, nonessential or unrecognized extensions on the Add-ons page. Either:

• Ctrl+Shift+a
• "3-bar" menu button (or Tools menu) > Add-ons

In the left column, click Extensions. Then cast a critical eye over the list on the right. All are optional; none are included with Firefox when you install it.

The cleaning programs mentioned in an earlier reply can help prevent reinfection.