What caused and how can I fix the brown leaves on my fir tree?

There are lots of reasons why this tree is turning brown. I would say it probably doesn't get enough water where it is located.

https://homeguides.sfgate.com/once-pine-branch-turns-brown-ever-green-again-100673.html

You can call your local extension service or the forest service to get the information you need. Forest service can indicate if you are getting the start of a bug kill or a spore in your ground, etc.

There are a number of pests that will damage evergreens. If you can trim a piece that is currently changing from healthy to dead, you might be able to identify if there is an insect (usually mite) involved. Take the piece and place it on white paper. Thump it and if anything (almost invisibly tiny) shakes loose, you have mites. If other trees are not involved, the mites could have been transported there by animals. You can treat for mites - check with a local nursery.

you do not say where you live? climate? So could be any number of reasons read this info. to help determine https://www.tulsaworld.com/lifestyles/home-and-garden/master-gardeners-brown-needles-on-pine-trees-have-many-causes/article_e9d8bcd1-2897-51b3-bae1-dd3357b893d3.html

It might be from a fungus. i would take a white piece of paper and hold it under it and shake it to rule out spider mites. But it looks like what happened to my Lelanld Cypress trees. If so. you need to cut out the dead branches and spray with a fungicide such as Daconil! If it is getting enough water, which looks likely that it is not, then proceed with what ever the County Extension Office advices.

I know in the Cascade Mountains we are having a very big problem with the Cedars, Hemlocks, and firs dying. I'm not sure if they understand why yet but, talk with your Forest Service to find out more about this problem.