This is a long imperfect record of my tests and results.  I just went to an empty desktop and pressed nothing but the buttons:
- keybind for iceweasel - super+W
- keybind for xfce4-terminal - super+T
- ALT+F4
in various sequences.  (In between this, I have to admit, I was doing CTRL+ALT+LEFT, typing in the browser text box to record, and doing CTRL+ALT+RIGHT again, and also maybe clicking or scrolling a few times.  So that could be a confounding variable.  But it certainly didn't seem to affect anything at least while I was doing the first 9 steps, which was quite a while.)

1 xfterm then iceweasel, proper behavior (raise and focus xfterm)
2 alt+f4 both
3 iceweasel then xfterm, proper behavior (raise and focus iceweasel)
4 alt+f4 both
5 repeat steps 1-2 many times, still proper behavior
6 repeat steps 3-4 many times, still proper behavior
7 repeat steps 1-4 many times, still proper behavior
8 repeat step 1 twice, then repeat step 2 twice, still proper behavior
9 repeat step 3 twice, then repeat step 4 twice, still proper behavior
10 do about 7 xfterm, then an iceweasel, proper behavior
11 alt+f4 all
12 do about 7 iceweasel, then an xfterm, proper behavior so far
13 alt+f4 3 or 4 times, then an xfterm, proper behavior so far
14 alt+f4 about 3 more times, then an xfterm - MISBEHAVIOR (xfterm was not raised or focused, stayed beneath iceweasel)

Now suddenly it was randomly misbehaving in ways that I couldn't predict or reproduce, and I began to lose track of exactly which combinations of steps I was doing.  For example:
15 open 3 xfterm, then an iceweasel - MISBEHAVIOR (iceweasel stayed beneath an xfterm)
16 alt+f4 all
17 repeat steps 13-14, proper behavior this time
18 keep repeating steps 13-14, proper behavior
19 do many iterations of various among steps 1-12, proper behavior throughout
20 repeat steps 12-14 - MISBEHAVIOR (exactly as before)
21 repeat steps 13-14, no misbehavior, totally proper
22 do a few iterations of various among steps 1-12, proper behavior
23 repeat steps 12-13 - MISBEHAVIOR (didn't even have to do 14 this time)
24 alt+f4 a few more times, xfterm, still MISBEHAVIOR, then alt+f4 all
25 various iterations, all proper behavior
26 repeat steps 12-13 - MISBEHAVIOR
27 various iterations, proper behavior
28 xfterm, iceweasel, xfterm - MISBEHAVIOR, alt+f4 all
29 xfterm, iceweasel, xfterm - proper behavior, alt+f4 all
30 various iterations, proper behavior
31 xterm, iceweasel - MISBEHAVIOR
32 many various iterations of 1-12, all proper behavior

Then I tested a generalized pattern to trigger, based on steps 12-14:
33 open 7 xfterms, then iceweasel - proper behavior
34 alt+f4 3 times, then iceweasel - MISBEHAVIOR
35 alt+f4 all, then open 7 xfterms
36 alt+f4 3 times, then open iceweasel - MISBEHAVIOR
37 open 2 xfterms, then alt+f4 once, then open iceweasel - MISBEHAVIOR
38 open 7 xfterms, then iceweasel - proper behavior, alt+f4 all
39 repeat step 37 - MISBEHAVIOR

Then I did all kinds of other things, not just with those keys, and then repeated step 37 again: still misbehavior.