What was the first ever first person shooter to feature a secondary fire mode? I can think of a couple of 1997 examples (Outlaws, Blood) but I wouldn't be surprised if it had been done a year or two earlier. I think Duke Nukem 3D had a secondary fire for throwing additional pipe bombs, but I'm not sure that really qualifies.
My answer is Star Wars: Dark Forces in 1995.
The game is often referred as a Doom clone, but it also added gameplay features that were uncommon to the first-person shooter genre at the time of release.
This comes from its original manual:
For combat, the player may use fists, explosive land mines and thermal detonators, as well as blasters and other ranged weapons. All weapons, with the exception of the fist, have a secondary mode which makes it have a different effect than in primary mode.