If there is a wall of fire up, is it considered an obstruction? Can a wizard cast fireball by choosing his point within range on the other side of the wall, or does the wall block his line of sight?
The Fireball would travel through, but with a caveat
"A bright streak flashes from your pointing finger to a point you choose within range..." PHB 241
The text for Fireball says nothing about LoS. It only specifies "a point you choose within range."
Wall of Fire:
"A creature takes the same damage when it enters the wall for the first time on a turn..." PHB 285
From this we can infer that creature and objects can pass through the wall. Therefore, the "bright streak" of Fireball would not encounter any physical obstruction and would easily pass through the Wall of Fire.
The description for Wall of Fire states:
"The wall is opaque..." PHB 185
meaning that no light can pass through it. In this situation, I would treat any target beyond or within the wall of fire as an unseen target and apply the rules for such:
"When you attack a target that you can't see, you have disadvantage on the attack roll." PHB 194
Though Fireball has no specific target but rather an area and does not require an attack roll, I believe it would be appropriate to give the targets advantage on the dexterity saving throw to achieve the same effect.
According to Roll20: https://roll20.net/compendium/dnd5e/Wall%20of%20Fire#content
The wall is opaque and lasts for the Duration.
So, a Wall of Fire would block line of sight. Your DM might let you cast a fireball blindly through the wall, perhaps with a check to locate the enemy and Advantage on their saving throw (since your aim would be off).
In general, yes
Physical objects like brick or glass provide total cover and you cannot target through total cover. However, a Wall of Fire does not provide total cover and targets on the other side are valid targets. Basically, if you could hit the target with an arrow you can target it with a spell.
Unless the particular spell adds the phrase "... that you can see" Wall of Fire is opaque so you can't see through it. Unless unless you can see the target in some other way (mirrors, Scrying etc.).
Spell Targeting (PHB p.204)
To target something, you must have a clear path to it, so it can’t be behind total cover. If you place an area of effect at a point that you can’t see and an obstruction, such as a wall, is between you and that point, the point of origin comes into being on the near side of that obstruction.
Cover (PHB p.196)
Walls, trees, creatures, and other obstacles can provide cover during combat, making a target more difficult to harm.
A target has total cover if it is completely concealed by an obstacle.
Wall of Fire (PHB p.285)
The wall is opaque and lasts for the duration.
Fireball (PHB p.241)
A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame
So the wall is merely opaque, it does not offer cover as it is not an "obstacle" such as a tree, creature or wall. It is not solid, merely opaque. Therefore the caster is free to target a point beyond it with a fireball spell, even though they cannot see that point through the wall.
To make this clearer consider an archer shooting arrows blindly through the wall. The wall provides no additional cover to the potential targets, the arrows go straight through it, it is merely opaque. What is applied are the rules for attacking a hidden target, rules that do not apply to Area of Effect spells, such as fireball, as they do not make an attack roll they target a point in space.
As a note, if there is an obstacle that the caster is unaware of beyond the wall of fire that intersects with the path of the fireball then it will will blossom into its explosion early at the point it meets the obstacle, potentially having undesirable consequences such as engulfing the caster.