No, Simulacrum, they're just 5 ft bursts which you can center anywhere you like within the usual rules for targeting.
Looking at the spells superficially firebrand doesn't look too bad. A 15th level caster casting firebrand gets 15 5 ft radius balls with a total area of approximately 1180 sq ft, while a 20ft spread has an area of approximately 1260 sq ft. A 15th level caster casting fireball (or delayed blast fireball) has a range of 1000 ft while the range for firebrand would be 250 ft.
Transferring real areas to squares changes the picture a bit. A 5 ft spread has an approximate area of 80sq ft, but looking at the area diagrams on 3.5 DMG p307 we see that when translated to squares each 5 ft spread has an area of 100 sq ft. This takes the total area of the 15 5 ft radius balls up to 1500 sq ft from 1180 sq ft. Looking at the diagram for a 20 ft spread the total area is 1100 sq ft down from 1260 sq ft. So the total area of a fireball shrinks by more than 10% while the total area of firebrand goes up by more than 25%. (As an aside, a think a fairer mapping for a 20 ft spread is for the "outer" ring with only 2 squares each to have 4 instead, which would make the area covered 1300 sq ft, closer to the real area of 1260 sq ft)
At 15th level would you cast delayed blast fireball to cover 1100 sq ft (using the DMG diagrams) with 15d6 damage (average 52) or would you cast an empowered firebrand to cover 1500 sq ft with 15 placeable 100 sq ft blocks for 15d6*1.5 damage (average 78)? Or at 20th level, would you cast delayed blast fireball to cover 1100 sq ft with 20d6 damage (average 70) or an empowered firebrand to cover 1500 sq ft with 15 placeable 100 sq ft blocks for 15d6*1.5 (average 78)? Unless you expect to be fighting at long range (ie/ more than 250 ft or so) it seems like the higher damage, greater area and ability to spread the area around selectively make the empowered firebrand a clear winner.
Moving on to chain lightning I'd agree with Simulacrum that it's not very good. At 15th level you get 15d6 on one target and 7d6 on 15 others. Averaging this out, we can almost discard the single 15d6 target and consider chain lightning to do 7d6 damage when cast by a 15th level caster. At 20th level this averaging effect is even stronger, with one target at 20d6 and another 20 at 10d6, giving an effective damage of 10d6 when cast by a 20th level caster. Chain lightning is long range, but that's about all it has going for it over the lower level firebrand. There may be some particular circumstances where chain lightning has an advantage, the only ones I can think of off the top of my head are higher save DCs and that it's the only 6th level spell that a 16th+ level caster can do more than 15d6 damage to a single target at long range, there are probably more. In general I think I'd favour a heightened firebrand over chain lightning and hope that I'd have enough mobility to make up for the shorter range of firebrand.