In mozilla 1.0 and NS 7.0, if you edit a plaintext file (e.g. mozilla -edit foo.txt), you get fixed-width font, no formatting toolbar, and lots of html-specific items greyed out of the main toolbar. Sometime since then, this has regressed: now I get a normal html composer window.
Regression, nominate nsbeta1.
Fixed as part of work on bug 169029