Frustrations with Zorin -

Once installed, you can then add it to the panel. Right click the panel and you will see a Panel option with a menu arrow- move to that arrow and you can then see "Preferences". Click that to open the Panel Preferences window. This has three tabs. The last tab is Items- these are the items added to or removed from the panel. Click on that Items tab and as you scroll through the list of available applets to add to or remove from the panel, you should see WhiskerMenu near the bottom.
You can click the Plus sign to add it to the panel. In this same Items tab, you will see on that right side toolbar an Up Arrow and a Down arrow. Highlight the WhiskerMenu in the items tab, then you can move it to the position you want it to be in using those arrows. Let's say you want to replace the Zorin Menu with it; Highlight the ZorinMenu Lite, then click the minus sign. Click whiskerMenu and click plus to add, then click the Up Arrow to move it to the top of the list, where ZorinMenu Lite previously occupied.

Gnome (Zorin Core) uses GDM3 to manage the Login Screen and GDM3 has the star wheel or cog wheel icon next to the User account.
Zorin Lite - XFCE4 - uses LightDM and not GDM3. The principles are the same, but the layout is just a bit different is all. Unlike GDM3, Lightdm has a Top Panel, much like the panel on your desktop (Taskbar in Windows). And this panel holds Items and tools. To switch desktop Environments from LightDM, look to the upper right side on that Panel and you will see keyboard icon, language icon... And Desktop Environment Icon, usually showing a Logo for that. If you are logged in currently on Zorin OS desktop Lite, then you will see the icon Zorin "Z" logo. If logged in XFCE, you may see the Mouse (animal kind of mouse) logo. Click that icon and a drop down menu appears showing your choices in Desktop Environment.

This is a known frustration to us all... Firefox is Very Persistent. Too persistent. Fortunately, there are solutions to that.:
Carmar and I both gave two different solutions to this:

When switching to something new and unfamiliar, getting your bearings is the toughest part. Like where that Star Wheel was and with a change to LightDM, the Desktop Option is in another place. How are you supposed to know where it went off to? With use, all things become familiar. With time, they become home.
At brief intervals per day, that could be a benefit. You can take your time and ask on Each Issue in a new thread, staying organized and getting step by step help along the way to familiarize yourself with the new Operating System and learn all it has to offer.