The reason is simple: CTRL-J is being used by FF as a shortcut key for download manager. And unfortunately, FF is not shipping a direct way for user to customize those keys. 'about:config' doesn't enable you to do that.
But there are still ways to tweak it:
1) With add-on. An unofficial FF add-on, 'keyconfig', is available at http://forums.mozillazine.org/viewtopic.php?t=72994. With that you are free to customize shortcut keys for FF, provided you know how to manually edit FF's user preference file (perfs.js). If you can't or don't want, install one more add-on, 'functions for keyconfig' from http://www.pqrs.org/tekezo/firefox/extensions/functions_for_keyconfig/index.html. This enables you to change keys through UI.
2) Without add-on. Knowing where to get a tool is good, but doing it w/o a tool is cool, right?
Let's look at how FF launches download manager first. Explorer %FIREFOX_HOME%\chrome, find the following 2 files:
Unjar browser.jar into %TEMP%, find browser.xul from extracted files and open it in your text editor, you see a line that looks like
- <menuitem id="menu_openDownloads" label="&downloads.label;" key="key_openDownloads" accesskey="&downloads.accesskey;" command="Tools:Downloads"/>
But what if you only want to change the shortcut key rather than disabling it? Let's look at how "key_openDownloads" is defined. In the same browser.xul, you see
- <key id="key_openDownloads" key="&downloads.commandkey;" command="Tools:Downloads" modifiers="accel"/>
Now extract en-US.jar into %TEMP%, find browser.dtd in which you see
- <!ENTITY downloads.commandkey "j">
Stuff the modified browser.xul or/and browser.dtd back into the jar files. Open FF and see whether it works.
This tip applies to both FireFox 2 and 3.