Line spacing tool works erratically

Line spacing tool works erratically

I'm running Zohowriter on Windows XP with Firefox and I find that it's not always possible to change the line spacing. You've programmed a default of 1.5 line spacing and I can't reliably change it to 1.0 line spacing. I highlight the text and click on the menu selection; sometimes it works, sometimes not.There's also a tendency for the program to revert to default. I've had similar problems with font family, size and style. Is this a Firefox compatibility issue?