Strange Response of a fir1 Filter in Matlab
Since you are seeing your filter coefficients and much larger than your signal, this is indicative that you have a very large "impulse" at the start of your signal. The coefficients of your filter is the impulse response for the filter, so that is exactly what you are seeing: the response to an impulse.

Review the start of your time domain data for a very large offset in the first sample or near the start of the file.