my fir trees are going brown what do I do?

If they're completely brown, then they're ex conifers, I'm afraid. You don't say how big they are, how long they've been planted, or what type they are, but most conifers do not regenerate from low down, only produce new growth at the top, so if they're already all brown but not dead, you will still have all that brown there even if they start growing again at the tips and tops of the branches.