I'm always late, but there are a few commands that could possibly help out, in order. Run these in a terminal, of course.

If there is something missing:
If there is something corrupted: I hope it says something useful.

@Powerwagon77 Thanks, did not even know about that.

I could probably add an option to do a sanity check with the update manager.

@ericbsd , that would be awesome.

As for @frogprince , if you see any output from it tells you what package is having problems, and you can run to force a reinstall and fix any issues.
HOWEVER, there are some packages that won't 'fix'. On my machine, libgdata and bluefish seem to have missing files, but it doesn't affect anything. I wouldn't go crazy trying to fix everything, just ones that look like they are part of your problem.
In your case, look at 'pkg info firefox' which shows you all the shared libraries needed.

HTH

So..running pkg check -d yielded nothing. Running pkg check -s yielded a semi-infinite list of packages whose signatures did not match. I ran fsck successfully, got a report of a clean system, but that did not fix the problem. Is there a way to force a complete update to the system without reinstalling?

"Semi infinate" made me chuckle, but I feel for your situation!

If you're willing, the commands below should be able to sort things out for you automatically. I have no idea how long this will end up taking. It could be quite a while, maybe even hours. It might be worth your time to just reinstall the OS. However...

This will put all of the problem packages into a file, removing duplicates: This will go through that file and reinstall those problem packages:

Powerwagon: thanks! But when I cut and paste the first line into a terminal as root, I get an error:

"Ambiguous output redirect"

Should there be spaces after all three |'s?

Well whatever it was... got me too. Firefox said it needed an update, and I selected firefox 61 in Octo and it did that and more, and more... I figured something was wrong when it continued. Must be a virus...
I'm going on vacation... can't worry about this for more than another hour. Then bye bye for several weeks.

Write me when you guys figure this out

Dave

I loaded Opra to get functional once again. However Java scripts are not turned on.... futzing with that now....what a waste of time.

Someone did this to us.... and I'm pissed.

Sorry guys, you're looking at my Linux roots showing. If you type 'bash', it'll switch your shell temporarily and those scripts will work properly. I'll work on getting them right for csh, tonight. They need to be run as user root.