Please move back to the 'I will consider fixing this' category.
Here's an updated version of the project with fixes to your issues (larger layout, collision detection removed). This file is a 600x500 layout, and no events (the ball just naturally bounces around the screen using the 'bounce off solid' behavior.)
It will take longer, but it still consistently (5 times in a row) disappears in Firefox (it just takes close to 5 minutes of bouncing before it happens.)
The original capx uploaded was a demo of the behavior. The original app was 640 x 480 pong game (I'm moving over to Construct2 from a competing product, and one of their examples on how to learn to use the system was to create a pong game, so I was in the process of recreating it using Construct2, and noticed that occasionally the ball would disappear/teleport instead of bouncing.) To demonstrate the behavior without forcing you to play a pong game for 20 minutes I constructed the attached capx file which shrunk the layout size considerably and inserted a 4th wall instead of a paddle.
As to the collision check, this is because the competitors product doesn't have a 'bounce off solids' attribute and I'm still learning Construct2. Nice addition 'bounce off solids' - very handy attribute.