I hate the three horizontal bars on top. Any idea on how to reduce or merge them like ubuntu 16?

You can turn off the titlebar in Firefox.

1. Press the Alt key and select View -> Toolbars -> Customize

   

2. Unselect Title Bar at the bottom of the page

   

3. Enjoy no Title Bar

   

There are several options.

1) Use the Pixel saver extension.

This extension will remove the legacy titlebar when an application is maximized. This will work nicely for Firefox, Libreoffice and any other application that uses legacy titlebars.

1. Install the extension with the command sudo apt install gnome-shell-extension-pixel.
2. Enable the extension using Gnome Tweaks or Extensions. Both are not installed by default, but can be installed with the command 1 sudo apt install gnome-tweaksorsudo apt install gnome-shell-extension-prefs`.

2) Hide the top bar

Hide the top bar. Then, only the chrome of your applications will be visible.

Install the extension: sudo apt install gnome-shell-extension-autohidetopbar. Enable the extension as outlined under 1).

You can combine this with option 1) to further increase vertical screen real estate.

3) Install Dash to Panel

You currently have launchers on the bottom. The extension Dash to Panel will combine these launchers with the elements of the top bar (application menu, clock, status menu, ...) into one bottom bar.

To install the extension, install gnome-shell-extension-dash-to-panel and enable it as described under 1).

You can go a step further and combine this extension with the "Pixel saver" extension to remove the title bar at the top when the application is full screen.

4) Only for Firefox: disable the titlebar

This removes the titlebar in Firefox only. Head to the menu (☰) and select Customize. In the left bottom corner, turn "Title bar" off.

There are Gnome Shell Extensions that provide extra functionality. To install Gnome Shell Extensions you will need to enable the Universe repository and you will need to install a few support packages.

First, run the following commands:

sudo add-apt-repository universe
sudo apt update
sudo apt install gnome-shell-extensions gnome-tweak-tool chrome-gnome-shell x11-utils

Then, you will need a firefox add-on to add extra extensions.

Navigate to the Gnome Shell Integration add-on website and install the add-on.

Finally, I haven't tested this particular extension out but I've used a similar extension in the past. The following extension is reportedly working on 20.04. If it doesn't work, you may be able to find a similar extension to do the same thing:

No Title Bar - Forked - Gnome Shell Extension

Also, you might want to search for something like "remove title bar gnome shell extension" if this one doesn't work for you.

After you install your extension, you can enable and manage your extensions through the Gnome Tweak Tool application which can be found in your list of installed packages. It is sometimes simply listed as "tweaks".