[ addendum: There are less... geeky... ways around it in the comments. Each has their own upside and downside. I added a filter to my chosen ad/popup/evilness blocker because it's what I already had running, and Privoxy works on MacOS, Linux, and Windows. Hopefully there will be a response to the suggestion that a switch be added. ]
Step 1: Install Privoxy. Follow the directions there. I can't help you set it up. It's pretty simple, if I recall correctly. It's nice to have, it does lots of ad-blocking and popup-killing stuff, too - it's a happy thing to have running.
Step 2: Edit the 'filter file'; you can find out where it is by going to http://config.privoxy.org/show-status once Privoxy's up. Use your favorite text editor.
Step 3: Paste this in:
# kill LJ's hideous new quick comment boxes
FILTER: lj-quick-comments Get rid of LJ's quick comment boxes
s/<a onclick='return quickreply\(.*\)' /<a /ig
and save the file.
Step 4:Go here and hit 'Insert new section at top'. Press 'edit' under 'actions' in that new sectton and hit the 'enable' button next to 'filter lj-quick-comments', submit that, then 'add' a new URL pattern of .livejournal.com. The dot at the front is, I think, important.
Works for me, at least. No more 'quick comment' boxes. I'd rather have a switch somewhere in LJ to disable them, of course.
Please don't ask me to help you set up Privoxy. I probably don't even use the same machine you do, and I had it running a good while back for ad-blocking.
[ Next day: Here's an improved hack, to make 'quick reply' a new option after the normal 'reply to this' URL. Use this instead of 's/<a onclick='return quickreply\(.*\)' /<a /ig':
s/<a onclick='return quickreply\((.*)\)' (.*)>Reply to this/<a $2>Reply to this<\/a>\) \(<a onclick='return quickreply\($1\)' $2>Quick reply/ig
You'll now have the (Reply to this) replaced by (Reply to this) (Quick reply), with the first link acting the old way.