node created 2020/03/15
Step 1: Locate and Open Profile Directory [..] Create a chrome directory within the designated profile directory which will host the necessary userContent.css file [..] In the address bar, open about:config and set the toolkit.legacyUserProfileCustomizations.stylesheets setting to true

Just think how needlessly obscure and ass-backwards that is.

Or take this:

I've been using Firefox's containers with Multi-Account Containers almost since the extension was released. One of things I did when I first started using it was set up containers for certain sites, obviously. However that left cookies and such for those sites laying around in the default container, still. So I went to `Settings -> Privacy & Security -> Cookies and Site Data -> Manage Data`, and was able to delete the default container cookies for each site individually while leaving the cookies for the site's specified container.

Today, I attempted to do the same. I created a container for a site, opened the site in that container, logged in, told it to always open in that container, then went to manage the data so I could delete the data from the default container. However, I see how way of doing that. In fact, the manage data popup doesn't even show to which containers each data belongs.

Was this functionality removed from Firefox? If so, why? If not, what changed, and what am I missing?

The responses are to either use third party add-ons, or:

You can delete individual cookies via the Storage Manager within Firefox's dev tools. Can't remember if the storage manager is enabled out of the box, you might need to enable it in dev tools settings.