There is a bug for this in SeaMonkey, but not for Firefox. Which seems strange. If there is a bug for this, I cannot find it. As is probably known to everyone there, one can use https to access google. This has led me to stop using the search field, though, as there seems to be no way to make that use the secure site. Looked in Security preferences to see if there is "Do Searches Securely (if available)" and I did not see it.
It also occurred to me to look in the list of search engines for "Google over SSL". And the "Manage Search Engines" pane. Nothing there. There are lots of places that this could be in the UI.