I'm not sure if it's intentional or incompetence but here's an example from Firefox: Count the number of actions it takes to delete specific cookies (say all matching "google", which I do every time I log into YouTube so it won't log me into Search etc.)
Firefox's UI is just appalling in a number of ways.
Ever since the new preferences dialog was introduced in Firefox, the UX has been quite slow. What was previously achievable through a few keystrokes now requires several.
For your use case, you could use the Cookie Auto Delete extension and configure it appropriately. Combined with Multi Account Containers and Temporary Containers, your life will be a lot easier.
Firefox's UI is just appalling in a number of ways.