The plug-in, developed by the German company Creative Software Solutions, is a tool for blocking malicious pages and phishing sites. Mozilla had previously recommended the plug-in on its blog.
"We've received concerns from the community about the Web Security extension and are currently investigating those concerns," a Mozilla spokesperson told British tech website The Register.
Fabian Simon, the managing director of Creative Software Solutions, told The Register that the plug-in only uses people's browsing history to check sites against Web Security's global blacklist.
"I am sure that if they look into the issue they will see that this is a normal and necessary behaviour," he said. – dpa