I was picking through a WordPress export file today and discovered some garbage in my database.  My immediate reaction was “Oh no, I’ve been hacked!”, but after a quick search, I found this can be the symptom of a user having the Firefox Browser Highlighter add-on installed.

<input id="gwProxy" type="hidden" />
<input id="jsProxy" onclick="if(typeof(jsCall)=='function'){jsCall();}else{setTimeout('jsCall()',500);}" type="hidden" />

The above was being placed in custom field set up as a WYSIWYG input area.

Asking the offending user to disable the plugin is one fix, alternatively there’s a FCK Firefox Fix Plugin available for WordPress.