Some unofficial installers bundle in a range of unwanted software and extensions.

To start, check the Windows Control Panel, Add/Remove Programs, and click the "Installed on" heading to view the most recent additions together. Anything unusual or suspicious there?

Within Firefox, try this:

Disable ALL nonessential or unrecognized extensions on the Add-ons page. Either:

• Ctrl+Shift+a
• orange Firefox button (or Tools menu) > Add-ons

In the left column, click Extensions. Then, if in doubt, disable.

Usually a link will appear above at least one disabled extension to restart Firefox. You can complete your work on the tab and click one of the links as the last step.

Does that eliminate or at least reduce the ads you see?

As an additional tool for mopping up unwanted software, try several of the scanner/cleaner tools in this article: Troubleshoot Firefox issues caused by malware. That also should help provide peace of mind that there's nothing else lurking.