McAfee SiteAdvisor lower than 4.0 has been blocked for your protection.

Why was it blocked?
Old versions of McAfee SiteAdvisor cause startup crashes starting with Firefox 48.0 beta.
Who is affected?
All users who have McAfee SiteAdvisor lower than 4.0. To resolve this issue, users will need to uninstall McAfee SiteAdvisor/WebAdvisor, reboot the computer, and then reinstall McAfee SiteAdvisor/WebAdvisor. For detailed instructions, please refer to the McAfee support knowledge base.
What does this mean?

The problematic add-on or plugin will be automatically disabled and no longer usable.

When Mozilla becomes aware of add-ons, plugins, or other third-party software that seriously compromises Firefox security, stability, or performance and meets certain criteria, the software may be blocked from general use. For more information, please read this support article.