Fire trap DOES benefit from spell modifiers, and it always had. The "spell" gem tag has always been there.
only for initial dmg, not the fireground burn
If you want your fire trap to do any good damage, you focus on initial hit damage and go crit so it would ignite on crit.
Its burning ground effect, while moderately strong, has often lagged behind in terms of base damage, unless you focused specifically on it.
It's a matter of choice, really - you either go multi-trap (as in using several different traps) and then you don't have enough passives to devote to dot for ftrap, or you go full derp with fire trap, and then you boost its base hit damage + aoe + dot/burn a lot, and you also go for crit so it would triple-dip on all the fire/ele damage nodes - first by critting with initial hit, then by igniting the target, and then by burning it with ground effect as well.
You could actually get a strong focus on fire damage instead of spell damage here, and use fire trap + firestorm trap + flame surge to cause maximum damage.
Sorta like this - tanky BM mutha marauder (also works well with the new ascendancy mara class that just got announced):
1) +2/3 fire gems and spell damage STAFF (you got it right, a staff! For extra 5-6L): fire trap + multitrap (or cluster trap upcoming support) + conc effect + fire pene + trap damage = RGGGB - 6th link options being empower and controlled destruction; for aoe mob clearing mostly.
2) carcass jack (for that aoe) OR infernal mantle (for that +1 gems): firestorm + trap + trap damage + conc effect + fire pene = RGGBB - 6th link option being controlled destruction; this is a single-target trap mostly, hence the absence of multitrap.
3) flame surge + echo + fire pene + conc effect/controlled destruction = RBBB - real good kill assist tool for tough single targets; can put it in voidbringer gloves or geofri's/the vertex if you're feeling adventurous.
...oh, and you could use iron will support gem for any of those as well, considering how much str you'd have. Or you could try to squeeze in the Repentance gloves instead - by using the astramentis ammy, for exampe, which would also fix all your dex requirements for gear and trap-related gems.
+2x 4L that you could fill with whatever. No need for any auras, so you could use one 4L for flame totem (bait for mobs + extra damage; ftotem + fcast + fproj + blind), and another one for utility like a movement skill + golem.
Would roll with this passive tree (mara + templar parts for BM + fire damage + aoe + life regen + delves into witch area a little for more of the same):
https://www.pathofexile.com/passive...VxpeVRtc6WFJTrJg9X5f08Ww22CGwSRs09yqNl4XsVS6U