I’ve noticed this issue running Firefox on Windows 10 ever since the update of Firefox to version 60 about a year or so ago. The issue was discussed in one or two threads around that time.
To get paragraph spacing to come out properly what I’ve had to do is to initially put a second space between paragraphs, then click the Preview button to see what that produces, and then usually for just some of the paragraphs eliminate the extra space based on what I see in the preview. Followed by an additional preview to verify that everything looks right. There seems to be no rhyme or reason as to where the extra spaces are necessary and where they aren’t.
Recently I’ve changed to Chrome, for reasons unrelated to Audiogon, and I don’t have that problem with it.
BTW, Uberwaltz, you’re probably already aware of it, but re Windows 7 I’ll mention that Microsoft will be discontinuing security patches and other forms of support for it in about 8 months, on 1/14/2020. It’s possible to purchase a W10 license and upgrade 7 to 10 without having to reinstall all of your other software and data, although a "clean install" of 10 followed by installation of programs and data from scratch might be preferable in terms of performance.
Best regards,
-- Al