Question: Just curious, but why install a 3rd party app to provide a service that's already built into the browser? You're having issues with their app, but you have not shown us anything. What OS? What version of Firefox?

Sorry about that.

I have a McAfee subscription until September 2025 and I use it on my laptop 64-bit on windows 11 without any problems.

My desktop has 64-bit windows 10.

I use the latest Firefox 133.0.3 (64-bit) on both versions!

I can't install it as a Firefox add-on on my desktop only.

I can't install it as a Firefox add-on on my desktop only.

What does that mean? Show us something, but this is something you will need to contact McAfee about because it's not Firefox issue that the add-on won't install.

I believe the current McAfee True Key extension has been blocked for violations of the Extension policies https://extensionworkshop.com/documentation/publish/add-on-policies/

Not the first time a McAfee extension got blocked as this old list shows https://blocked.cdn.mozilla.net/