I have firefox installed with sudo apt install firefox but after sometime a new snap version seems to automatically be installed. Maybe after update.
And now there is two icons for firefox. Same happened when I used flatpak version of firefox. It even removed all data from flatpak version.
Why is this happening? I only want apt version.
Go to software store and in installed apps, find Firefox, there should be two versions there in the dropdown. Apt and Snap. Uninstall the Snap version.
Welcome to the forum! 
I did but it reinstall again. it is just a guess but I think this happens when there is a new update or if I update the system.
Open terminal:
sudo snap remove --purge firefox
No i mean I do know how to remove it. But it shouldn't even be installed in the first place automatically without even asking.
and that code removed even apt firefox not just snap 
How strange that it removed the apt version as well. 
Even more strange that both versions appear to be in synch together.
What a dilemma. My apologies for my experience here being very limited.
Hopefully one of the big pros here will see this soon and help you out in this situation. 
I don't think it removed completely but after that the apt icon still was there but was not opening. So I ran sudo apt install firefox again and now it's working.
Thanks anyway.
Welcome to the Forum!
Yes, that is a known Problem. This is related to Ubuntu because Ubuntu itself doesn't offer an APT Version for Firefox. Only Snap. And they created a Redirect when You use the apt install Command to install the Snap Version.
On Zorin there should be a Redirect, too what would add the Mozilla-own Repo and then install the .deb Version of the Browser. With that, there should be landed a File in /etc/apt/preferences.d/ which is simply called mozilla. In this is set the Priority for the Mozilla Source.
To make that better, You can in this set down the Ubuntu Source. Type in Terminal:
sudo nano /etc/apt/preferences.d/mozilla
and in the upcoming Window add that:
sudo apt update && sudo apt upgrade
Let's call this a solution. It hasn't installed snap firefox yet. but I guess I'll need to wait for an update to check.
Hi and welcome.
As an alternative you could go to mozila's Firefox page and install from a tarball instead. Updates to it will then be via Help | About, just like in that other OS:
Donct forget to download the Unofficial Manual for Zorin 18 Core, still a work in progress: