The key
attribute is a reference to a XUL <key>
element that specifies a keyboard shortcut for that menuitem.
In this, case, the add-on contains:
<key id="selenium-builder-run-key"
modifiers="accel alt"
key="B"
oncommand="bridge.boot()" />
Which means a <menuitem>
with key="selenium-builder-run-key"
can be activated by pressing "Control-Alt-B" on the keyboard (or Command-Opt-B on a Mac).
As for your other question, searching for that ID in an old version of Firefox finds it in one of the files included by browser.xul
, but it looks like it's been removed in newer versions of Firefox.