User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:72.0) Gecko/20100101 Firefox/72.0

Steps to reproduce:

Before Firefox updated this morning, this selector in userChrome.css correctly hid the top tab bar:

#main-window:not([tabsintitlebar="true"]) #TabsToolbar {
visibility: collapse !important;
}

Actual results:

I was just updated and now that selector no longer works. I tested and removing #main-window:not([tabsintitlebar="true"]) fixes it, but obviously doesn't do what I want. I further found that box #TabsToolbar is the highest level ancestor that works -- body #TabsToolbar, html #TabsToolbar, :root #TabsToolbar all do not work.

Current version: 72.0b8 (64-bit) Firefox Developer Edition, aurora update channel
OS: Linux

Expected results:

I should be able to create userChrome.css selectors across the html:body boundary in order to make them only apply based on attributes on #main-window/:root/html.

userChrome customizations are not supported. Ask for help on a user forum like https://www.reddit.com/r/FirefoxCSS/

Figured it out! All I had to do was remove @namespace url("https://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); from the top of my userChrome.css file.

Comments in that Reddit group tipped me off to this: https://www.reddit.com/r/FirefoxCSS/comments/brmi8v/psa_firefox_v69_users_will_have_to_set_a_pref_to/eu2apsn?utm_source=share&utm_medium=web2x

See also https://www.userchrome.org/adding-style-recipes-userchrome-css.html#namespaces