Firefox will block the unsafe add-on from loading and present you with the following message:
To prevent these add-ons from running, click Restart Firefox.
When you try to install an unsafe add-on that is on the blocklist, Firefox will block the installation and display the message:
Firefox can prevent a plugin from running automatically (which prevents many issues). When Firefox blocks a plugin you'll see a message similar to this:
You can then choose to run the plugin or update it (if an update is available). To learn more, see Why do I have to click to activate plugins?
Click the menu button and choose Add-ons. The Add-ons Manager tab will open.
To check if any of your installed plugins are out of date, visit Mozilla's Plugin Check & Updates page.
If you were blocked from installing an add-on, check to see if there is an updated version of the add-on you were trying to install.
See the list of blocked add-ons.