Sometimes a problem with Firefox may be a result of malware installed on your computer, that you may not be aware of.

You can try these free programs to scan for malware, which work with your existing antivirus software:

• Microsoft Safety Scanner
• MalwareBytes' Anti-Malware
• TDSSKiller - AntiRootkit Utility
• Hitman Pro
• ESET Online Scanner

Microsoft Security Essentials is a good permanent antivirus for Windows 7/Vista/XP if you don't already have one.

Further information can be found in the Troubleshoot Firefox issues caused by malware article.

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If you get a pop-up message asking to update Firefox or plugins then such a message is likely a scam and you should never respond to such an alert to avoid getting infected with malware.

• Only update Firefox via "Help > About" or by downloading and installing Firefox from the Mozilla server and never via a pop-up or link on a web page.
• Plugins should only be updated via the plugin itself or by visiting the home page of the plugin.

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

Not a problem caused directly by Firefox.

It is most likely because of something you did, or did not do; possibly after visiting some dodgy site, or opening some social engineering webmail post.

The last two posts are replies aimed at helping you to sort this out.

Separate issue; Your System Details shows;

Installed Plug-ins

Shockwave Flash 14.0 r0 Shockwave Flash 13.0 r0

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

Nothing to do with the problem. The pop ups are definitely advertising spam generated via Firefox.

Eg : Ads By Plus-HD-V1.9

This is not in the Firefox browser install nor Mozilla's doing. Firefox from mozilla.org does not come with any form of ads, virus, malware, toolbars or such as Mozilla would lose far to much for what little gain by doing such.

It is likely either due to malware on computer and or one or more Extensions you have installed.

This "Ads By Plus-HD-V1.9" may be a extension that got installed on Firefox (did not come with Fuirefox or from Mozilla).

May not be the best guides but for example http://www.anvisoft.com/resources/how-to-remove-brought-by-plus-hd-v1-6-ads-adware-removal-guide/ http://www.411-spyware.com/remove-plus-hd

Bing search;

Plus-HD-1.6 by Kimahri Software inc - Should I Remove It? http://www.shouldiremoveit.com/Plus-HD-81475-program.aspx

What is Plus-HD-? Plus HD (distributed via FIRSERIA) is a web browser extension and Browser helper Object that delivers contextual based advertising to the web browser. In addition it will modify the user's browser home and search pages as well as 'New Tab' pages to push advertising and search. The software acts as an adware type application and is typically defined as a unwanted application by various malware vendors. The software is marketed as a variation of DivX Plus HD for the purposes of getting users to download even though it is not in anyway associated with DivX.