- in a new tab, open "about:config" and search for "http://www.google" (search box is located at the top of the tab area). You 'll find "keyword.URL" setting and you can change its value by double-clicking on it. Just add "s" after "http".
- open bookmarks by pressing Ctrl+Shift+B and search for "google.com/search? q=" (search box is located at the upper right corner of the window). At first, you can eliminate all duplicates (as I did), then change all "http" occurrences to "https"
For Google Toolbar users, I believe there will be an update very soon.