I came across a pretty good video detailing the seam sealer along the firewall of the 1980-1997 F-Series trucks and Broncos. I personally will be doing what was mentioned in the video, as the seam sealer on my 1990 cab is long gone, and I can almost guarantee that yours is too. You may not have visible rust, but if your seam sealer is dry and pulling away from the panel joints, water is 100% getting in there, and the rust process is already starting.
https://www.youtube.com/watch?v=t42tgkjmics