Project causes serious lag/freeze in Firefox

[kchadha]

https://llk.github.io/scratch-gui/develop/#71816138

@kathymakes notes in bug hunt on 10/12/18:

When the cloned cats turn a corner or get shot by the arrow, it lags the project (Windows firefox)

Kevin notes (in the same bug hunt):

On Mojave/Firefox, playing a few rounds of #71816138 causes browser tab to freeze. No console output giving any indications. (kevin)

I am also seeing this issue on Mac OS Firefox (right as I was about to lose the game?) Game gets completely stuck in this state and then the yellow warning/error bar pops up on the top of the webpage:

[Mr-Dave2]

The issue might be how good is your aim and how fast can you click. The bullets clone themselves and are overlapping so you can't tell. However, If there are 5 bullets on the screen and you click the background you'll create 16 more clones in a frame and each one will then play a sound and start running several stacks.

I remixed the project to spreads out the bullets. https://scratch.mit.edu/projects/262733430/ Scratch 3 link: https://beta.scratch.mit.edu/#262733430/

For me, when the total clone count is 15, my Scratch 3.0 Firefox session often locks up. (have to click fast). Also, I always test this in full screen mode. It looks like my changes to show the individual bullet clones has made this lockup problem easier to reproduce. I have only tried this on Windows 10 Pro 64-bit, Firefox 63.0.1 (64-bit).
Intel® Core™ i7 -8700K Processor 16GB NVIDIA GeForce® GTX 1070Ti PCI Express

[thisandagain]

/cc @mzgoddard

[williamhorning]

I can replicate this on a laptop using Firefox 81 on Windows 10 Enterprise build 19041 with an Intel Core I5-6200U and 8.00 GB of ram.