It could be because I set something up incorrectly, but people have been seeing unusably narrow text entry boxes (TEXTAREA) in Movable Type 2.51 with Mozilla (1.0 and 1.2.1), Opera 7 and IE 6.0. So I ran:
mkdir new && for i in *.tmpl; do cat $i | sed -e \<br />
's/<TMPL_IF NAME=AGENT_MOZILLA> cols=\"\"<\/TMPL_IF>/\<br />
cols=\"80\"/g' > new/$i; done && cp new/* ./<br />
in the tmpl directory and it all seems to be usable now. This is probably not the Right Way™ but it makes the entry boxes usable.
By the way, if you want to add code snippets to your Movable Type postings, just use