Type the following command in the terminal and then simply focus the application whose title bar you want to remove

Hide title bar:

xprop -f _MOTIF_WM_HINTS 32c -set _MOTIF_WM_HINTS "0x2, 0x0, 0x2, 0x0, 0x0"

Show title bar:

xprop -f _MOTIF_WM_HINTS 32c -set _MOTIF_WM_HINTS "0x2, 0x0, 0x1, 0x0, 0x0"

Note: These changes are usually temporary and will revert if the application is closed and reopened, or if your session restarts. You can maybe try assigning keyboard shortcuts to these commands. I haven't found an alternate fix.

Remove firefox title bar

For firefox you can remove the title bar from its settings instead of the above appraoch:

• Click on the "Hamburger" button (three horizontal lines)
• Go to "More Tools"
• Click "Customize toolbar"
• Uncheck title bar

References

• https://forums.linuxmint.com/viewtopic.php?t=300908
• https://forums.linuxmint.com/viewtopic.php?t=359607