That’s not how the math works. The synergy bonuses are multipliers, 1.4x damage with a maxed orb synergy is always 1.4x damage. It doesn’t matter how many points in Cold Mastery you have.
If you have 1*1.0 vs. 1*1.4, the 1.4 increases it by a factor 1.4. If you have 2*1.0 vs 2*1.4, the 1.4 still increases it by a factor of 1.4. Having Cold Mastery doesn’t magically make it a 1.8 multiplier.
He’s level 68, if he maxes Ice Bolt he’ll have hardly any points for Firewall and his Firewall will be fairly weak. If he was level 90, yeah sure, maybe maxing the orb synergy makes more sense.
Maximum Firewall damage skill point allocation at various +skills with 21 points to spend in Warmth/Fire Mastery:
+0 skills
7 Warmth, 14 Fire Mastery
Damage bonus: (1 + 7*0.04)*(1.23 + (14 + 0)*0.07) = 2.8288
Compare to 20 Warmth, 1 Fire Mastery
Damage bonus: (1 + 20*0.04)*(1.23 + (1 + 0)*0.07) = 2.34
You gain 2.8288/2.34 = ~21% damage doing the optimal allocation.
+5 skills
9 Warmth, 12 Fire Mastery
Damage bonus: (1 + 9*0.04)*(1.23 + (12 + 5)*0.07) = 3.2912
Compare to 20 Warmth, 1 Fire Mastery
Damage bonus: (1 + 20*0.04)*(1.23 + (1 + 5)*0.07) = 2.97
You gain 3.2912/2.97 = ~11% damage doing the optimal allocation.
+10 skills
12 Warmth, 9 Fire Mastery
Damage bonus: (1 + 12*0.04)*(1.23 + (9 + 10)*0.07) = 3.7888
Compare to 20 Warmth, 1 Fire Mastery
Damage bonus: (1 + 20*0.04)*(1.23 + (1 + 10)*0.07) = 3.6
You gain 3.7888/3.6 = ~5% damage doing the optimal allocation.
The more +skills you have, the more points you’ll want in Warmth. And the smaller the difference you’ll see between the optimal allocation and all points in Warmth first. But at low +skills, like where OP is at, you can see a fairly sizable increase in your Firewall damage by optimizing the skill point allocation. For example at +5 skills (it looks like this is where he’s at with dual Spirit + Lore), he’d see about an 11% increase in his Firewall damage with 9/12 instead of 20/1.
You could definitely argue the utility of Warmth outweighs the damage. But 11% more Firewall damage is also decent. Will help with the cold immunes as he progresses through Hell.