It can sometimes be difficult to tell if a web page or domain a link points to is a legit site or not. While the url of the site may give you clues at times, for example if you know it already, it is at other times not possible to determine whether it is secure to visit a site or not based solely on a link.
One of the things that you can do in this case is to check the site against malware databases. There are plenty of services available on the Internet that can do so, from Google's Safebrowsing to Web of Trust.
Many of those have in common that they provide you with one result. If you prefer multiple results instead, as it improves the odds of the assessment being right, then you can use services such as Virustotal instead.
Just visit the service, click on url, paste the link in that you want to check and wait for the results to be displayed on the screen. While certainly useful, doing so is not really suitable if you need to check links regularly.
That's where the Firefox add-on Security Plus comes into play. The browser extension for Firefox adds a new entry to the right-click context menu to check the underlying link on Virustotal.
This is done without leaving the site you are on as results are displayed right on the page in a small overlay on the screen.
Even better, you can check links in rapid succession with results of all those checks displayed on the page.
The results indicate whether the linked site is clean or not. You can click on the results link on the right of each report to display the list of engines used in the check and how they rate the link.
Security Plus uses Virustotal which means that it will use all 64 services supported by Virustotal to scan the selected links.
The browser extension requires that you launch the scan manually as there is no option included to scan links automatically instead. So, it is best used to scan questionable links that you come across while you are browsing the Internet.
The main advantage over the official Virustotal extension for Firefox is that the result is displayed on the same page and not on a new page that is opened when you initiate the scan.