> There are no obvious ways to actually install scripts

Script installation works by loading URLs that end up on ".user.js". This can be done by clicking a link or manually entering the URL. You can also drag and drop files ending up on ".user.js" to Firefox. Finally you can use the "Utilities" tab, enter a URL at the "URL" section and click "Import".

> The support site doesn't quite work (has a frozen pop-up on top)

Do you have JavaScript blocked/disabled i.e. via NoScript?

> but what shows gives Chrome specific instructions

This is on my TODO list. However, many answers apply regardless of browser in use.

> That brings the question: what are the developers trying to hide and why?

Either there is nothing hidden besides the concrete implementation or neither the Mozilla Add-on review team, nor the Opera extension review team, nor the Windows Store team did their job.

> There is no way somebody could abuse such a license and benefit from it.

In a perfect world: yes.

Please try to disable the internal linter:

"Config mode" -> "Advanced" and then disable "Automatic syntax check on typing"

If this doesn't help you can also try to disable the option "Highlight trailing whitespace".

Config mode -> Advanced,

Please try the latest version from dev channel.