fir tree disease

Question

needles turning brown & falling off.

Answer 1

Needles drying and falling off is a dire sign, especially if the entire foliar canopy is affected.

It is most likely a water related problem, too much or too little. Too much can cause root rot. Too little water creates drought stress, which can result in branch die-back and mortality.

There are pest and disease issues that affect fir trees, but close inspection is required for diagnosis Or good macro photos of leaf and twig details, bark of the trunk to look for holes from beetle/borers as well as wide angle photos of the entire tree, and also the site with surrounding plantings.
I would suggest calling your local extension agent and give him or her more details.
https://www.gardeningknowhow.com/extension-search