Check your Browser Extension Preferences   for pop-ups, as sometimes when you download things and click Agree, then you are inadvertently allowing certain applications to install malware onto your computer.
 

To remove extensions on Firefox :
 

1. Open Firefox .
    
2. Click on Firefox   > Preferences.
   
    
3. Click on the Privacy & Security tab.
   
    
4. Scroll down to Permissions and open Exceptions.
   
    
5. Click on the Unwanted or Suspicious Extension(s), and select Remove Site on the bottom left.