Question on syntax?

Scripting Forums AppleScript | Mac OS X
#1

This works:

tell application "System Events"
	set arrURL to value of UI element 1 of combo box 1 of toolbar 2 of first group of front window of application process "Firefox"
end tell

set arrURL to domain for arrURL

but this fails:

tell application "System Events"
	set arrURL to domain for (value of UI element 1 of combo box 1 of toolbar 2 of first group of front window of application process "Firefox")
end tell

My translation of the error message: “The type for:(value of UI element 1 of combo box 1 of toolbar 2 of first group of front window of application process Firefox) must no be after domain.”

What is the difference? How do I write that in one line?