Please check Norton Toolbar won't work with Firefox version 41. The problem can be also caused by new add-on signing procedure introduced in Firefox 43 - in that case, please contact Norton support, if they do have an update including signed add-on (it was announced by Mozilla a long time ago, so they should have it).

Anyway, the add-on itself does not have much impact on your computer security. Antivirus can check most of the network traffic as well as the browser behavior and files written to disk even without any add-ons installed in Firefox.

https://community.norton.com/en/forums/norton-toolbar-limited-compatibility-patch-firefox-43-available https://community.norton.com/en/blogs/product-update-announcements/firefox-43-limited-compatibility-norton-toolbar

The Norton toolbar needs to be updated for each new major Firefox Release and has done so pretty much ever since Firefox 5.0 Release.