Hi,
today i updated firefox to version 43.0.4 and now is norton family add-on (version 3.5.0.43) was disabled by firefox, because add-on isnt signed for use in firefox.
Please solving this problem for all users , we need secure sign this add-on.
info about firefox sign add-on
https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox
How to fix it. But little bit unsecure.
Override add-on signing (advanced users)
You can temporarily override the setting to enforce the add-on signing requirement by changing the preference xpinstall.signatures.required to false in the Firefox Configuration Editor (about:config page). Support is not available for any changes made with the Configuration Editor so please do this at your own risk. Signing will be mandatory with no override, in Firefox 46 beta and release versions. For details, see this Mozilla blog.
Many thnx for solving this GLOBAL SECURE problem.
Laxxman