Can I assume these pages contain advertising or some other content not from your history and not associated with any programs you use? Maybe the sites provide a clue as to the culprit.

If this only happens when Firefox is running:

I would suspect an add-on. Try disabling ALL 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, 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.

Any improvement?

If this can happen with Firefox completely closed:

Scan for malware using some of the tools listed in our support article: Troubleshoot Firefox issues caused by malware.

There might be an additional step:

Last year, many users reported advertising tabs opening randomly, sometimes even when Firefox was closed. It turned out that a program had created scheduled tasks to open those pages at certain times of day.

You can check the Windows Task Scheduler:

• Windows Vista/7: Start button > Control Panel > System and Security > Administrative Tools > Task Scheduler

• Windows XP: Start button > [All] Programs > Accessories> System Tools > Scheduled Tasks

• Past threads on the "dsite" problem:
  • How can I keep Firefox from opening a window to website tofushopnews.com
  • How do I get rid of sponsorship tabs that open automatically?

Do a malware check with several malware scanning programs on the Windows computer. Please scan with all programs because each program detects different malware. All these programs have free versions.

Make sure that you update each program to get the latest version of their databases before doing a scan.

• Malwarebytes' Anti-Malware:
  http://www.malwarebytes.org/mbam.php
• AdwCleaner:
  http://www.bleepingcomputer.com/download/adwcleaner/
  http://www.softpedia.com/get/Antivirus/Removal-Tools/AdwCleaner.shtml
• SuperAntispyware:
  http://www.superantispyware.com/
• Microsoft Safety Scanner:
  http://www.microsoft.com/security/scanner/en-us/default.aspx
• Windows Defender:
  http://windows.microsoft.com/en-us/windows/using-defender
• Spybot Search & Destroy:
  http://www.safer-networking.org/en/index.html
• Kasperky Free Security Scan:
  http://www.kaspersky.com/security-scan

You can also do a check for a rootkit infection with TDSSKiller.

• Anti-rootkit utility TDSSKiller:
  http://support.kaspersky.com/5350?el=88446

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked

Your System Details shows;

Installed Plug-ins

Adobe PDF Plug-In For Firefox and Netscape 11.0.07 Adobe PDF Plug-In For Firefox and Netscape "9.5.5"

Having more than one version of a program may cause issues.