Browser detection incorrect

I am on Windows 10. I do have some security settings set (these settings). Will not let me install an add-on even though I do have a valid Firefox version

There’s an issue open for that: In the meantime, the only workarounds are to download and install the XPI manually, or revert that pref change when you need to install something from AMO.

Ok, thanks @jorgev. Just to be clear for future searchers - this is the privacy.resistFingerprinting setting, correct?

Yes, that’s the pref causing the problems.

Since AMO has privileged access to browser information, would it be possible to look beyond the UA string to determine the “actual” version?

For security and web compatibility reasons, the privileged access AMO has is limited to what it strictly needs, and we are trying to reduce that even further rather than add things to it.