If you have installed the latest update from Firefox' Add-ons menu and tried to write jQuery commands into the console/command line of firebug you might have run across an annoying problem.
Firebug tries to autocomplete
$(', etc. adding $ and completing ' after the next character. Basically you have to keep hitting escape or delete to write your jQuery statement.
This problem is luckily fixed in the latest beta release. It is not available through the add-on menu in Firefox, but can be downloaded directly from the Firebug website. So head over to http://blog.getfirebug.com/2010/12/22/firebug-1-6-1b2/ and download it.