Snapshot of window in Polish language attached.

If you do not remember ever installing Firefox, perhaps it was bundled with something you downloaded?

Go to the Windows 7 Control Panel, Uninstall a Program, click the "Installed on" column heading, and look at your recent installs to see whether you can determine how it got onto your system.

That also would be the place to uninstall it.

Or if you want the English language version, scroll down on this page for the correct installer:

https://www.mozilla.org/en-US/firefox/all/

Considering the possibility that your computer is now controlled by people in faraway lands, you should run some supplemental malware scans. This article has recommendations for tools: Troubleshoot Firefox issues caused by malware.

Last but not least, look in the Windows 7 Task Scheduler to find tasks that may be running software at midnight or other times. To access that, either:

• Start menu > All Programs > Administrative Tools > Task Scheduler
• Control Panel > search box: task sch > click Schedule Tasks

You can 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.

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: Home Page:
  http://www.microsoft.com/windows/products/winfamily/defender/default.mspx
• 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