Let's start with something very basic, and that is, you do not need to use the paste button on most websites. The button just reads what is on your clipboard and sticks it into the form. You can do that yourself using Ctrl+v or right-click>Paste.

For your security, I suggest using those standard Windows keyboard shortcuts (Ctrl+x cut, Ctrl+c copy, Ctrl+v paste) or the context menu.

Occasionally you will find a paste button that runs a clean-up script or otherwise does something useful. That is where the (admittedly a bit complicated) instructions come in handy.

To open your active profile folder, you can use:

Help > Troubleshooting Information > "Show Folder" button

Which help article are you using for the modifications?

Finally, please be cautious in opening up your clipboard to sites. You may have stuff you copied from other pages or other programs that you do not want to share with most sites.

Typically when a user is directed to the Mozilla Rich Text Editing Midas Demo webpage, there is an issue with the lack Copy /Paste permissions in Firefox for the domain. If you click the edit your preferences file hyperlink on the Demo page you are directed to Setting Prefs for the Mozilla Rich Text Editing Demo.

If that is what you want to do, there is an extension which will create those permissions a lot easier than doing it manually thru the creation and editing of a user.js file yourself.

AllowClipboard Helper
https://addons.mozilla.org/en-US/firefox/addon/allowclipboard-helper/

Manages security policies in user.js file with a convenient user interface...