A few people have complained about Preview Mode defaulting to being on. It took me a while to work out where the bug was… Sorry (Sheepish grin)!
Right after upgrading to 3.1.0.1 the pref for Preview Mode doesn’t exist in the Firefox prefs. Incorrectly, the Wizz RSS code is assuming that if the pref doesn’t exist, then Preview Mode should be on.
Quick fix: open the Wizz RSS Options page and just click the OK button.

Thank goodness for that!
@Helma: I Wizz RSS users to get involved with beta testing new versions. If more users were willing to get involved with testing, rather than having the attitude that testing is someone else’s problem, this kind of thing wouldn’t happen.