So I've got a guy who's playing an "elemental" character with the Insubstantial 3 (Fire-Energy) power. He already has Fire Immunity 5, so when he "flames on" does the Fire-immunity of the Insubstantial power stack with his previous fire damage immunity, thus granting the elemental the equivalent of a complete immunity to Fire effects?
No, your Immunity ranks do not stack with Insubstantial ranks for extra degrees of Immunity.
5 ranks of Immunity would make a hero either immune against certain non-damage effects, or immune against damage effects with one chosen descriptor.
5 ranks: alteration effects, sensory Affliction effects, emotion effects, entrapment (grabbing, snares, or bonds), fatigue effects, interaction skills, or a particular Damage effect, descriptor (such as bullets, cold, electricity, falling, fire, magic, radiation, sonic, etc.)
Immunity against all fire effects would require 10 ranks.
10 ranks: a common power descriptor (such as all effects with cold, electricity, fire, radiation, or weather descriptors, for example)
In case the Immunity power's wording is ambiguous, below are examples from the Power Profile documents, demonstrating some expected applications of the Immunity power effect.
From the Fire Powers document:
Immunity to Heat: Immunity 1 (Environmental Heat), Immunity 5 (Heat Damage), or Immunity 10 (Heat Effects) • 1, 5, or 10 points
From the Speed Powers document:
Frictionless: Immunity 5 (grab and entrapment effects) • 5 points
From the Talent Powers document:
Unfazeable: Immunity 5 (Interaction Skills) • 5 points
Meanwhile, using Insubstantial 3 (Fire Energy) means that the hero's body is made of fire and becomes immune to both physical damage and fire attacks.
Rank 3 - Energy
You become coherent energy. You have no effective Strength, but have Immunity to Physical Damage. Energy attacks (other than the energy making up your form, to which you have Immunity) damage you normally. You can pass through solid objects permeable to your type of energy, but energy resistant barriers, like heavy shielding or force fields, block your movement.
Immunity against Fire attacks would protect the hero against any Attack-type power (either Affliction, Damage, Nullify, Weaken), or anything else that requires an attack roll, with the Fire descriptor. But they may be vulnerable against other fire-based hazards or effects.
By default, Immunity is a permanent effect, and Insubstantial is sustained. When used together, Immunity 5 (Fire Damage) and Insubstantial 3 (Fire Energy) are mostly redundant, and redundant abilities do not usually stack to provide a greater degree of benefit.
Note that Insubstantial 3 (Fire Energy) costs 15 power points, whereas Immunity 5 (Fire Damage) costs only 5 power points; hence the overlap in defenses.
However, if the hero spends 5 Power Points, then they can increase Immunity 5 (Fire Damage) to Immunity 10 (Fire Effects).
I feel obliged to add to Mike Q's excellent answer. I do agree with him that in general Immunity does not stack, but provides a series of discrete categories that may overlap. However, Immunity is a power specifically noted to be very subjective, based on the utility in the game in question, which gives your GM fairly broad purview in how they adjudicate the power. In a situation where it might make sense, they might allow it to be additive, such as the example of someone with a 5 PP Immunity to Fire Damage who has an Alternate Form that uses those 5 PP, plus 5 more, to gain Immunity (Fire). I would, however, caution GMs from allowing this as a matter of course, particularly when it comes to "bundled" powers like the Energy Form's immunity to attacks of its own descriptor, as part of their discount involves them being tied to other aspects, which makes them less broadly useful (for example, enemies generally assume hitting The Human Torch with flamethrowers is useless, so it comes up less often).
The other area where I disagree is the standard list of 5 PP immunities. Within the latest release, the Basic Heroes Handbook, a comma has been eliminated, bringing Immunities back to the 2E definition:
5 ranks: Alteration effects, dazzle effects, emotion effects, entrapment (grabbing, snares, or bonds), fatigue effects, interaction skills, or a particular Damage effect descriptor (such as bullets, cold, electricity, falling, fire, magic, radiation, sonic, etc.)
That pretty clearly indicates that the only item in the list that require a particular descriptor is the Damage effects (which are some of the most common effects), which matches official builds that simply include Immunity (dazzle effects) or Immunity (entrapment) as 5 PP immunities.