Without a specific example I'm speculating a bit here...

Most browsers allow pages to detect when the user is leaving the page and run a script before leaving (using the beforeunload event). It's useful in some cases, for example, where the page lets you edit something and you never saved your edit.

You might be able to block scripts from hooking into that event (I'm not sure it's possible, but there's probably a way), but you might not be able to block it in one certain circumstances, e.g., going Back versus loading another page in the same tab versus closing the tab.

If you get it all the time then you may have caught malware or a Firefox extension.

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Do a malware check with some malware scanning programs on the Windows computer.
You need to scan with all programs because each program detects different malware.
Make sure that you update each program to get the latest version of their databases before doing a scan.

• http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware
• http://www.superantispyware.com/ - SuperAntispyware
• http://www.microsoft.com/security/scanner/en-us/default.aspx - Microsoft Safety Scanner
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page
• http://www.safer-networking.org/en/index.html - Spybot Search & Destroy

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

• http://support.kaspersky.com/viruses/solutions?qid=208280684

See also:

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