ToME: the Tales of Maj'Eyal

Right now I'm playing an archmage focusing on fire damage. I have a lot of effects that put fire on the ground, so I've got a pretty high fire resist (to stay alive).

If I invest in some fire damage penetration gear, am I in effect lowering my own survivability?

yes

but archmage gets a talent that makes them immune to their fire damage

Surround yourself with Wildfire, increasing all your fire damage by 3.4%, 6.2%, 8.4%, 10%, 12%, ignoring 20%, 34%, 43%, 50%, 56% fire resistance of your targets and reducing self-inflicted fire damage by 18%, 36%, 54%, 72%, 91%.