According to the Question Details you have Windows 2000 (NT 5.0), but that Windows version wouldn't be able to run Firefox 19 without special precautions.

• http://www.mozilla.org/en-US/firefox/19.0/system-requirements/
• http://www.mozilla.org/en-US/firefox/12.0/system-requirements/

Are you running Firefox by any chance in compatibility mode on a more recent Windows (XP) version?

Check that you do not run Firefox in compatibility mode or with a reduced screen resolution.
Make sure that you do not run Firefox in compatibility mode.
You can open the Properties of the Firefox desktop shortcut via the right-click context menu and check the "Compatibility" tab.
Make sure that all items are deselected in the "Compatibility" tab of the Properties window.

What OS are you really using as Firefox 12.0 was the last to work on Windows 2000 without doing some workarounds like in http://forums.mozillazine.org/viewtopic.php?f=23&t=2482475

Your useragent and system details says you have Win2000.

You need at minimum Windows XP SP2 since Firefox 13.0

You may be running Firefox in compatibility mode.

Thank you, that was the problem. Upgraded Firefox. Another has popped up. Now when i click on some tabs, including my e-mail an ad will show up...sometimes with audio

Thank you, that was the problem. Upgraded Firefox. Another has popped up. Now when i click on some tabs, including my e-mail an ad will show up...sometimes with audio

Hi moltar, to help assess whether an extension is responsible for the ads, try visiting those same sites in Firefox's Safe Mode. That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Diagnose Firefox issues using Troubleshoot Mode.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

Do a malware check with some malware scanning programs on the Windows computer.
Please scan with all programs because each program detects different malware.
All these programs have free versions.
I'm not sure if they all work on your OS.

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/
• 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