What you can try to do is install an extension such as Ad-Block Plus, https://addons.mozilla.org/en-US/firefox/addon/adblock-plus, which will block many different forms of advertising.

Also, update your Firefox to version 12, and update your plugins to make sure you stay on top of security threats :) http://www.mozilla.org/plugincheck/.

If that fails to work, there is a possibility you might be infected with viruses, try Download and Install MalwareBytes Anti-Malware, run a full Scan. http://www.malwarebytes.org/

Download and Run TDSSKiller http://support.kaspersky.com/faq/?qid=208283363

Download and Install Microsoft Security Essentials http://windows.microsoft.com/en-US/windows/products/security-essentials (not an official endorsement, but I personally recommend MSE as an awesome permanent anti-virus)

Double check for all Windows Updates.

this sounds like your pc might have got infected with malware or some other sort of unwanted software. here are a few recommendations:

• in firefox go to addons > extensions and disable or uninstall all of your extensions except the AVG ones & SmartWebPrint from HP
• in firefox > addons > plugins disable all the following unusual plugins (unless you know that you've installed them intentionally & trust them): Roblox Launcher Plugin, SOE Web Installer, ijji Web Launching Plugin for FireFox, nprt
• also go to the windows control panel's program section and uninstall all toolbar-software that you have not installed intentionally or do not use anyway.
• run a full system scan with your AVG security software already in place & use the tools that tyler has already mentioned above.