Being in CA myself, it's extremely hard to estimate what the negative effects of the Firestation will have. Seeing as though many homes are within a short stone's throw of "soon to be developed" commercial land in the major metro areas, this is always taken into consideration.
Coming from a Mortgage Broker / Realtor's point of view, faced with this dilema, I would inquire with several Appraisers.
Any appraiser worth his salt in CA will have faced these issues before and know how much of an adjustment would be required. The proximity of the firestation will have to be noted on any Freddie/Fannie appraisal form so there is no overlooking it.
Seeing as though the last two people in line in the transaction who will utimately decide fair market value are the appraiser and the lender's underwriter, an appraiser would be the best person to ask.
No matter what anyone says, if it isn't an all cash transaction, the local market and consumer may direct pricing, but the underwriter, with the input of the appraiser, has the final word on actual value. Sorry guys, I'm a Real Estate Agent as well and no, we don't have final say.
We all live and die by the underwriter and noone wants to go back to the seller and say the "appraisal came in low".
Ask a couple of local Residential Appraisers who follow USPAP. If you can't find one, look on:
http://www.orea.ca.gov/ .