Note, I have discovered a workaround with which I can live in the process of writing this email. I've decided to complete the email so that someone else having a like problem might find this.
I have created a new site using a codebase shared with others.
I have created two input formats General All roles may use default format configure Wiki authenticated user, Committee configure delete
For some time I was fixated on these being at the heart of my problem, but it's not so.
I have created a user account, santa, with no special permissions, but then created the roles Committee (which you see there) and Power, with permission to do everything.
Using another browser, SeaMonkey 1.1.14, I logged on as Santa and created a blog entry. I have subsequently editted it a few times.
The first problem I noticed is that the menu for "Input format" does not expand.
Some time later I saw that "Attached Images" and "Book Outline" do not expand either.
While composing this email, I logged onto another of my sites, one where I have created content successfully in the past and discovered the same problem.
My conclusion is that the version of Seamonkey I am using is incompatible with Drupal 6.x. I do have Javascript enabled
Just possibly, the problem arises because I have denied cookies: while loading the page in question, I get two messages on the Seamonkey error console.