Here's a tip to make using Firefox profiles easy.

1. go to about:profiles and create a profile. Let's call this one "better" because it has the Betterfox css added. (an example)
2. On Linux typing firefox in a terminal will launch the default profile. Typing firefox -p better will launch the "better" profile. If you're on OS X or Windows just find out what the equivalent command would be to execute the launch.
3. Go to wherever you need to in your operating system to create custom keyboard shortcuts. For me that's Settings > Keyboard > Keyboard Shortcuts > Custom Shortcuts. Create a new shortcut with the following information.

Name: Firefox profile better

Command: firefox -p better

Keys: enter the keys you want to use

How to choose keys;

My desktop environment uses super(windows) + b to launch the default browser, which happens to be Firefox. So for me it makes sense to use something like super + shift + b for consistency, and easy to remember. If you have a profile with Arkenfox css it could be super + shift + a. Regardless, try for a mnemenic and consistent solution.

Now whenever you want to launch a new instance of a Firefox profile just tap the keys you assigned.