Hello

 I'm sorry to hear that you're having an issue. Let's see if we can resolve that issue.  Are the pages that automatically open advertisements? If so I would start with installing the ad block plugin available for Firefox. This should only allow the websites that you invoke to open. Secondly, are you running SP3 with your Windows XP? There are quite a few security updates as well as stability updates integrated within SP3 that may resolve these type of issues. Sometimes these issues can be resolved by simply reinstalling Firefox. If you choose the latter, please make sure that you export your bookmarks, plugin's, etc prior to doing so.

 I hope this helps

Additionally, please also check if this happens in Safe Mode.

Problematic Extensions

Troubleshooting Extensions and Themes

Uninstalling Add-ons

Uninstalling Toolbars

Safe mode disables the installed Extensions, and themes (Appearance) in Tools (Alt + T) > Add-ons. Hardware acceleration is also temporarily disabled - the manual setting is Tools > Options > Advanced > General > Use hardware acceleration when available. Options > Advanced. All these settings/add-ons can also be individually or collectively disabled/enabled/changed in Firefox normal mode to check if an extension, theme or hardware acceleration is causing issues.

Options

Another thing you can try doing is scanning for Malware:

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.

What is the website that opens up? Is it always the same, always different, one of a few, etc?