I found out about it yesterday. When you press it, the cursor will move to the first text input area in the web page.
It's completely wiped from my mind now though.
Note: I am using windows 7
It seems that Fox Input add-on could do the job: https://addons.mozilla.org/en-us/firefox/addon/fox-input/
Otherwise, there shouldn't be a native shortcut to do the task for FF. The list of FF shortcuts can be found here: