Q1) Do domes, shield shapes and bubbles make the cut vs firearms based on their structure?
To examine this, it's pretty easy to look at breastplate designs after firearms became a popular weapon. You'll notice that the breastplate is not a simple curve, but has a central ridge. The reason is to try and divert a projectile from its most probable trajectory. You're much more likely to be attacked from the front, so having armor in a wedge shape was important to deflect the projectile from the center of the body to the side. This comes at a risk (shots coming from slightly to the side will hit more straight on), but most shots most the time will have their effectiveness reduced.
Q2) Can anyone tell me if there is a physics-based reason why these shapes/structures might be so popular in the first place?
No, of course not. It just looks cool, and it makes simple sense. Since magical force barriers have never been battle tested, those criteria will determine what's going to be most popular, even if different shapes would actually be more effective.
Q3) Is there any way magical shields could be improved by altering their shape/structure alone?
Absolutely. The problem with a sphere/bubble is that any attack hitting it dead on with have to be stopped instead of deflected, and deflecting an attack takes a lot less force than stopping it. Further, a sphere/bubble with it's center on the person is most likely to be head dead on a lot, since people will be aiming for... the person in the middle.
So, if the wizard knows where the attack is coming from, s/he can work to deflect it. If you've read any of the Dresden Files books, you'll notice Harry Dresden frequently uses his shields in a flat angle to deflect attacks coming from a specific direction. If you're getting attacked from absolutely every direction, a sphere can make some sense, but that is rarely the case. This means you can make shields designed to deflect attacks just enough to miss you from the direction you expect.
You can also look at how fortifications changed following the rise of cannons. Straight and curved walls were terrible at deflecting cannon blast, so people started building star forts. This isn't a perfect transfer (much of the star fort design is to channel charging infantry into kill zones), but you can see how a modified star shape would be able to more effectively channel projectiles away from your body than a sphere, even when being attacked from every direction. The points of the stars can protect you from side shots that would threaten a simple wedge, and the shots that do hit a plane head-on aren't actually pointed at your body, so if they break through you still aren't dead.
The biggest danger with the star shape is the concave points. But, since you aren't actually worried about charging infantry, you can have deliberate gaps here that lead to another interior star barrier that still channels projectiles away from your body. That way, there's no point in the shield that needs to stop a projectile, just redirect it.
Seen here, the circle is the mage, and there are two nested six pointed stars. The best design would require some more design work and careful consideration of the angles (and, therefore, number of points), entry point size, and if projectiles that entered the first barrier could be redirected to exit again, instead of needing to be absolutely halted by the internal barrier. But this is a good place to start.