ActiveX controls are another term for Add-ons that we see associated with browsers like Internet Explorer, Firefox etc.
Other terms: Browser extensions, browser helper objects, or toolbars etc.
Add ons improve your experience on a website by providing multimedia or interactive content, such as animations. However, some add-ons can cause your computer to stop responding or display content that you don’t want, such as pop-up ads.So, you have to select the right Add-on application to feel the good browsing experience.
PS:You may check the official links for downloading Add-ons from top rated browsers like IE,Firefox and Chrome here.