Not a simple user-friendly dialog, but the add-on explains it well enough that anybody who isn't totally computer illiterate will be able to perform this.
Just make sure that nobody is shoulder-surfing when running the script.

Even following bruceybonus's steps to get this working, I still get an error with this extension:
"cannot run in this environment, try error console"

This is on Firefox 103.0.2 on macOS

There are a couple things you need to do
- in console type allow pasting before pasting (ignore error about "Uncaught SyntaxError: unexpected token: identifier")
- use about:config and set general.config.sandbox_enabled to false
- The generated script has a wrong line on line 2 it should be typeof(Cc)
not typeof(Ci)
- afterwards toggle general.config.sandbox_enabled back

Thank you for making this possible, though folks at Mozilla (sorry!) are making it difficult/impossible to do it otherwise.

The process is a bit convoluted but it actually works! I think the extension should require fewer permissions 'though

It is a very good addon. This stopped working on latest version of firefox. Gives an error. Running 66.0.3 (64-bit) - The Components object is deprecated. It will soon be removed. debugger eval code:1:4

Thanks Dmitry, your addon saved my life after my first periodic password change with Quantum FF.
Considering the absence of comparable addons and the technical hurdles you had to tackle with Quantum this is quite an achievement by you.

Pretty simple. Follow the instructions and it works like a charm.