You can open the about:config page via the location bar and do a search for jzip via the Filter at the top of the about:config page.
You can reset all jzip related prefs that appear bold (user set) via the right-click context menu to their default values.

• http://kb.mozillazine.org/about:config

Thanks - this seems essentially the same as is contained in https://support.mozilla.com/en-US/questions/805889#answer-161093, which I found eventually by doing a google search on "jZIP". 161093 is rather more informative than this answer, which is pretty cryptic, though the ref to Mozillazine is helpful.

I was nervous about setting anything to default (... here be dragons...) , but after uninstalling the jZIP toolbar program and pasting my homepage address into the pref that jZIP toolbar had taken over, all is well.

Thanks to all concerned.