Losing space in disc Ubuntu 20.04

I have an issue that I would like to understand and resolve: why am I losing space in my machine?

I am using Ubuntu 20.04 and I have a total space of 99Gb. However, I noticed that in the last two/three days I lost almost 4GB in the space available (see pics below, /dev/nvme / -first line). I did not download anything, I just used Firefox and Thunderbird email. So I do not know what is going on.

I read some threads where people checked the details of df -h (my output below) but I do not know what to do from here to solve my problem.

So, I wonder: What could be the root of this issue? and how can I solve it? Any suggestion would be highly appreciated.

Output df -h:

Filesystem      Size  Used Avail Use% Mounted on
udev            7,8G     0  7,8G   0% /dev
tmpfs           1,6G  2,0M  1,6G   1% /run
/dev/nvme0n1p5   93G   16G   73G  18% /
tmpfs           7,8G  944K  7,8G   1% /dev/shm
tmpfs           5,0M  4,0K  5,0M   1% /run/lock
tmpfs           7,8G     0  7,8G   0% /sys/fs/cgroup
/dev/loop1      100M  100M     0 100% /snap/core/11187
/dev/loop0      208M  208M     0 100% /snap/code/67
/dev/loop3       55M   55M     0 100% /snap/core18/1880
/dev/loop2       56M   56M     0 100% /snap/core18/2074
/dev/loop4       66M   66M     0 100% /snap/gtk-common-themes/1515
/dev/loop5       63M   63M     0 100% /snap/gtk-common-themes/1506
/dev/loop6      256M  256M     0 100% /snap/gnome-3-34-1804/36
/dev/loop7       50M   50M     0 100% /snap/snap-store/467
/dev/loop8       33M   33M     0 100% /snap/snapd/12159
/dev/loop9       51M   51M     0 100% /snap/snap-store/547
/dev/loop10      30M   30M     0 100% /snap/snapd/8542
/dev/loop11     219M  219M     0 100% /snap/gnome-3-34-1804/72
/dev/nvme0n1p1  256M   34M  223M  14% /boot/efi
tmpfs           1,6G   36K  1,6G   1% /run/user/1000

Pics

I'd like to say what I think.

1. It is normal to use the browser and email client to bring some cache files. They need to cache some login information, pictures, videos and even some static files to speed up the access. This takes up some disk space.
2. It may be an automatic security update to the system that downloads some packages accidentally, but I recommend that you don't remove them without fully understanding them.

If you really want to clean up, there are many ways to do it online. For example: sudo apt autoremove removes packages that are no longer needed. You can also download specialized cleanup software, such as BleachBit

I think cache files, logs, and automatic updates and package downloads are the main reasons for your reduced free space.