Assuming that you're not referring to the firewall on your computer (which I assume you have to block out unwanted intruders such as those who would try to convince you that Firewall -- the movie -- is an excellent movie sans flaws.  Or an actual sans attempting to get past your firewall.  A sans is far worse than a worm because it is "without" mercy.  At this point, if anybody reading this is completely lost, I advise you to look up the word, "sans" -- unless you are sans a dictionary) that empty feeling you've mentioned after watching that particular film could be the popcorn.  There's something they're putting on movie popcorn these days that could be on the Top Ten List Of Things Not To Let Into Your Body Or You'll Get That Empty Feeling (Or Worse).

I truly enjoyed Firewall (not the one on my computer -- because I'm always in fear of it not being able to block out a sans) and, limiting my comments to the script, being someone who is a script consultant and therefore limits his comments to scripts, it was a perfectly executed formula script, following the prescribed thriller well-trodden path admirably.  If there was any "flaw," if one can call it that (I can) is that the screenplay may have followed it too closely (if you know what I mean.  Wink, wink; nudge, nudge.  I have no idea why I just did that unless I have a developing tick and a spastic elbow).  Everything worked out perfectly even to the point that...

*WARNING*

At this point, anybody who hasn't seen the film and doesn't want to know about a remarkably profound plot point in the third act that allows the protagonist to eventually overcome that smarmy, faux-sophisticated villain be advised that that very plot point will be revealed now.

... the protagonist is able to find the location of his captive wife and daughter and eventually single-handedly overcome the antagonist and his motley crew because of his dog's collar (I won't give the full reason to continue the suspense for those who haven't seen the film).  Brilliant!  The greatest strength is that, even within the formula, the writer does some very nice legerdemain (another word I force you to look up because I like to look erudite by using big words instead of using a better known word with the same meaning) with the storyline, keeping us off-balance and, therefore, more involved as we try to decipher what we're observing.

That empty feeling you've referenced ("reference" is a relatively new word used by those who think "referred to" is beneath them and they want to appear like they've had more education than they really have), Frank, may correlate with the rather less than emotionally cathartic moment of the viewer when the status quo returns at the end of the film.  Many of the audience did let out a collective "uuggghh" when the dastardly and very driven bad guy found the center of his back entered by a pick masterfully wielded by the hero, but I'm not sure we all felt that "collective" about having a thoroughly satisfied and resolved relief when the hero and his family are spotted coming over a hill, disheveled, stressed, but alive and well.  All I can say is that, as good as the film is, something is missing, that often inexplicable "something" that you know the film needs even though you can't put your finger on it.  That "something."

Could it be that Firewall with all its well-crafted tension and action, is still a little transparent.  And what do we see when we look closely?

That darn formula.

DcH