Double title bar in Firefox

[Thomasster]

Distribution (run cat /etc/os-release):
NAME="Pop!_OS"
VERSION="20.04 LTS"
ID=pop
ID_LIKE="ubuntu debian"
PRETTY_NAME="Pop!_OS 20.04 LTS"
VERSION_ID="20.04"
HOME_URL="https://system76.com/pop"
SUPPORT_URL="http://support.system76.com"
BUG_REPORT_URL="https://github.com/pop-os/pop/issues"
PRIVACY_POLICY_URL="https://system76.com/privacy"
VERSION_CODENAME=focal
UBUNTU_CODENAME=focal
LOGO=distributor-logo-pop-os

Related Application and/or Package Version (run apt policy $PACKAGE NAME):
firefox:
Installed: 75.0+build3-0ubuntu1
Candidate: 75.0+build3-0ubuntu1
Version table:
*** 75.0+build3-0ubuntu1 500
500 http://us.archive.ubuntu.com/ubuntu focal/main amd64 Packages
100 /var/lib/dpkg/status

Issue/Bug Description:
Double title bar appears in Firefox.

Steps to reproduce (if you know):
Hamburger button > Customize... > uncheck "Title Bar"

Expected behavior:
The "main" title bar disappears and the close button appears in the same line with the opened tabs.

Other Notes:
Happens after clean install, there wasn't any previous Firefox profile setup.