what does "Your browser is being managed by your organization" mean?

Firefox offers a lot of policy controls intended for businesses that are running Firefox for their employees. That message would mean that one of those are enabled. However, it doesn't necessarily mean that you've setup a policy yourself. For the average user, it usually means that your antivirus or security software as enabled a policy. Generally speaking, they enable a policy that allows them to import their own security certificates into Firefox (the <code>ImportEnterpriseRoots</code> policy). If you are curious, you can see exactly what policies are enabled on the <code>about:policies</code> page in Firefox. Hope this helps.