No! FIR filters are NOT always linear phase. The acronym 'FIR" only tells you that the impulse response is finite in duration, nothing more and nothing less. So called "minimum phase" FIR filters are not linear phase. (A tapped-delay line, lowpass, FIR filter whose coefficients are [0.5, 0.7, 0.9, 0.8, 0.55] has a finite duration impulse response but it's NOT linear phase.) Also, phase linearity does NOT depend on the length of an FIR filter's impulse response. FIR filters using real-valued coefficients have linear phase if those coefficient values are exactly symmetrical. FIR differentiators & Hilbert transformers using real-valued coefficients have linear phase if those coefficient values are exactly antisymmetrical.