At random times while using Firefox, Firefox would say the website isn't responding and will close itself. Afterwards, all the icons on the Zorin Desktop goes missing (some folders and shortcuts to apps) and clicking on the Firefox taskbar icon doesn't work.
Only a restart of the laptop will solve this issue.
How much RAM do you have? It sounds like the system is starved for free memory, and is killing processes to keep from crashing.
If so, how much swap space do you have? sudo swapon --show
If none, get a swap drive set up. I know nowadays many say it's not needed, or only a small 2 GB swap is needed. I still go with 2x your RAM as minimum swap size. I've actually got 4x, across two swap drives. I've tried crashing the system by consuming all memory... it doesn't happen, the system slows down a bit, but just keeps going.
You could try nohang:
nohang/focal,focal,now 0.2.0-1~oibaf~f all [installed]
sophisticated low memory handler for Linux
So it shows that your system is using the swap file... usually my system doesn't even touch the swap, it stays at 0 bytes, even when I've got SRWare Iron (Google Chrome with all the corporate spyware stripped out) open with many tabs. And that's with 'swappiness' set to 100.
That could mean that when Firefox is open, it's consuming all the free memory, and your swap drive is too slow to keep up, so it crashes (that's the reason I have two swap drives... one drive still caused crashes when I stressed memory because the drive was too slow to keep up... the kernel expects quick access to the data)... in that case, nohang will help.