Thanks for all the replies.
Yes, that should work. In fact, that's what I was doing on the laptop before I discovered the Force Content Type addon.

But it's remarkably tedious - even on a laptop with a mouse and with all the normal linux facilities for renaming files and dragging them into the browser.

I was hoping for something easier. It doesn't need all the facilities of Force Content Type - All it needs is to change the content type of every input site to text/html. I would have thought a few lines of userchrome would do it - but I'm guessing.

[I would normally discuss that on Mozilla usenet but this proxy doesn't allow NNTP - and nobody will take any notice if I post with GoogleGroups - even if that works :-( ]