I have the same issue since one or two months, I’m on Firefox Nightly 142.0a1 currently.

For me it looks like this:

Firefox on the left, Dolphin (which uses the system titlebar control buttons) on the right.

A few months ago, firefox also used the system titlebar control buttons. When I noticed the change at first, I also searched for solution online and in about:config, but didn’t find anything. All other solutions posted here sadly don’t work:

• browser.tabs.inTitlebar only adds a standalone titlebar, like you noted.
• When searching for non-native in about:config, I don’t see any titlebar buttons option that I can turn off.
• Vertical Tabs are already disabled for me in the settings.

If anyone finds a solution to this, I would be happy to be notified. Thanks in advance!

They’re asking about the password prompt for the disk encryption, which is shown before the rootfs can be accessed. Thus, installing a display manager to the rootfs will not help. Furthermore, a display manager serves the purpose of logging in users, not unlocking an encrypted partition.