Pop-ups can appear from clicking any links within Firefox. I have reinstalled Firefox and ran SpyBot scan, but they never stop.
Looks like your machine is infected with some adware or malware. I would suggest you to perform a full scan using AdwCleaner and Malwarebytes Anti-malware free. It can help remove hard to find adware, malicious software and potentially unwanted programs.
- Download AdwCleaner and run It. Click a Scan button. Once the process completed click a Clean.
- Download Malwarebytes Anti-malware and run. Follow the Anti-Malware Setup Wizard steps. Perform a full scan. When the scan is done, click Remove selected.
- Reset Firefox, follow the steps http://www.myantispyware.com/2016/02/07/how-to-reset-mozilla-firefox-settings-to-default/