Firefox window decoration - titlebar

A weird window decoration problem occurs with Firefox when launched in a maximized state with Title bar disabled. It opens fine, but once resized it has this odd extra decoration on the window (see the picture). The bug doesn't go away when maximized again and occurs for all new windows launched in a maximized state.

When Firefox operates with Title bar enabled, it behaves as expected.

I have Global Menu and the WIndow Picker applet in the top panel. The Global Menu applet crashes often when I switch applications quickly. The window decoration problem occurs on Marco with or without Compton, as well as on Compiz.

I’ve reported this on Mozilla’s Bugzilla.

The weirdest thing is that on my main system it doesn’t happen. If I clone my whole home directory to a new user, it still won’t happen, but I haven’t been able to track it down to one setting or config file. It’s not just my Firefox profile.

Until it's fixed you can use devilspie to keep firefox undecorated.

Interesting. Please, let me know if you manage to narrow it down to a particular cause

Thanks. I think for now I will just keep Firefox with a Title bar and then disable window decoration for maximized windows in MATE Tweak. The WIndow Picker applet and keyboard shortcuts take care of closing, minimizing and restoring maximized windows then.

This still happens to me in Ubuntu MATE 19.10...
Have anyone found a workaround?

Have you tried on 20.04? What about Linux Mint MATE - does it happen there too?

In UbuntuMATE 20.04 LTS it still happens.

Guys, I think I found a workaround.
In about:config I set true for both options which appear on searching "titlebar". So now with no titlebar there are no double-window issues for me.
The options are

Hope this helps!

The culprit here is the Maximus Window Placement Manager and removing the package mate-netbook (sudo apt remove mate-netbook mate-netbook-common) will fix this. Just to be on the safe side, re-enable Firefox Title Bar CSD, and reboot.