I ran my nitrous lines through my firewall and i removed that grommet to run my lines and didn't bother to put it back in. I didn't think to much of it until the next time it rained and water ran down into my floorboards. I can't find the grommet and im not sure if would really help anyway. Has anyone else had this problem and know how to fix it.

 

put the gromet back in and make a loop so that the water drips off the line before going into the firewall. rtv doesnt hurt either

 

If you can't find the grommet, go to your local auto parts store (I used Advance Auto) and look at their PVC grommets. They come in all sizes, you just need to know what diameter the hole is (approx).
I also agree with the loop. I run cable wiring for satellite dishes, and I always do this. The RTV is a good a precaution also.