On *nix - edit /usr/share/firefox/chrome/toolkit/content/mozapps/profile/profileSelection.xul around line 91

<listbox id="profiles" rows="10" seltype="single"
    ondblclick="onProfilesDblClick(event)"
    onkeypress="onProfilesKey(event);">
</listbox>

On Windows you have to jump through a few more hoops. Go to \Program Files\Mozilla Firefox\chrome and unzip toolkit.jar, then edit content\mozapps\profile\profileSelection.xul the same as above. I also changed the dialog style (around line 60) to read:

  style="width: 30em; height:400px;"

Then re-zip using store instead of deflate and replace the existing toolkit.jar file.

You can also download my firefox-2-toolkit.jar, if you don't want to do it yourself. All Firefox sessions have to be closed in order to replace toolkit.jar.

• Set the environment variable MOZ_NO_REMOTE=1 (My Computer->Properties->Advanced->Environment Variables
• Create new profiles with "firefox -profilemanager"
• Run "firefox -P profilename"

[ Network services ]
* Looks for an Internet connection.
* Connects to "lazy.irwanmartin.com" on port 6667 (TCP).
* Connects to IRC server.
* IRC: Uses nickname |521508.
* IRC: Uses username mlraczsp.
* IRC: Joins channel #bin with password 64b5e2Nb.
* IRC: Sets the usermode for user |521508 to -x-i.