Every time I log in, for some reason, I get a little box that pops up in the middle of my screen that says:

invalid option: -session

It just started yesterday and I'm not sure what's causing it. I'm running Xubuntu 12.04 64-bit.

This sounds as if something went wrong in some startup scripts. They are usually in /etc something with init or some such.
Did you make an update recently?
You can check the updates with
sudo grep -Ei "upgrade" /var/log/dpkg.log*

When I execute that command it comes up with a lot of information. Not sure how I'm supposed to post it here.

Yeah, sorry. I don't upgrade much. So either you use somethink like pastebin.
Or better just shorten the command with tail:
sudo grep -Ei "upgrade" /var/log/dpkg.log* | tail -30
Will print the last thirty lines. Adjust so that the day before this first occured is in the list.

EDIT: Additionally there are more logs in /var/log/ and /var/log/apt/

/var/log/dpkg.log.1:2013-06-26 12:19:42 upgrade ooobasis3.4-testtool 3.4.1-1 3.4.1-1
/var/log/dpkg.log.1:2013-06-26 12:19:43 upgrade ooobasis3.4-writer 3.4.1-1 3.4.1-1
/var/log/dpkg.log.1:2013-06-26 12:19:44 upgrade ooobasis3.4-xsltfilter 3.4.1-1 3.4.1-1
/var/log/dpkg.log.1:2013-06-26 12:19:44 upgrade openoffice.org3 3.4.1-1 3.4.1-1
/var/log/dpkg.log.1:2013-06-26 12:19:45 upgrade openoffice.org3-base 3.4.1-1 3.4.1-1
/var/log/dpkg.log.1:2013-06-26 12:19:46 upgrade openoffice.org3-calc 3.4.1-1 3.4.1-1
/var/log/dpkg.log.1:2013-06-26 12:19:46 upgrade openoffice.org3-draw 3.4.1-1 3.4.1-1
/var/log/dpkg.log.1:2013-06-26 12:19:48 upgrade openoffice.org3-impress 3.4.1-1 3.4.1-1
/var/log/dpkg.log.1:2013-06-26 12:19:49 upgrade openoffice.org3-math 3.4.1-1 3.4.1-1
/var/log/dpkg.log.1:2013-06-26 12:19:50 upgrade openoffice.org3-writer 3.4.1-1 3.4.1-1
/var/log/dpkg.log.1:2013-06-26 12:19:51 upgrade openoffice.org-ure 3.4.1-1 3.4.1-1
/var/log/dpkg.log.1:2013-06-26 21:50:59 upgrade thunderbird-globalmenu 17.0.6+build1-0ubuntu0.12.04.1 17.0.7+build1-0ubuntu0.12.04.1
/var/log/dpkg.log.1:2013-06-26 21:50:59 upgrade thunderbird-locale-en 1:17.0.6+build1-0ubuntu0.12.04.1 1:17.0.7+build1-0ubuntu0.12.04.1
/var/log/dpkg.log.1:2013-06-26 21:51:00 upgrade thunderbird 17.0.6+build1-0ubuntu0.12.04.1 17.0.7+build1-0ubuntu0.12.04.1
/var/log/dpkg.log.1:2013-06-26 21:51:02 upgrade thunderbird-gnome-support 17.0.6+build1-0ubuntu0.12.04.1 17.0.7+build1-0ubuntu0.12.04.1
/var/log/dpkg.log.1:2013-06-26 21:51:03 upgrade thunderbird-locale-en-us 1:17.0.6+build1-0ubuntu0.12.04.1 1:17.0.7+build1-0ubuntu0.12.04.1
/var/log/dpkg.log.1:2013-06-29 22:08:42 upgrade libldap-2.4-2:i386 2.4.28-1.1ubuntu4.2 2.4.28-1.1ubuntu4.3
/var/log/dpkg.log.1:2013-06-29 22:08:43 upgrade libldap-2.4-2 2.4.28-1.1ubuntu4.2 2.4.28-1.1ubuntu4.3
/var/log/dpkg.log.1:2013-06-29 22:08:47 upgrade chromium-browser-l10n 25.0.1364.160-0ubuntu0.12.04.1 28.0.1500.52-0ubuntu1.12.04.2
/var/log/dpkg.log.1:2013-06-29 22:08:48 upgrade chromium-codecs-ffmpeg-extra 25.0.1364.160-0ubuntu0.12.04.1 28.0.1500.52-0ubuntu1.12.04.2
/var/log/dpkg.log.1:2013-06-29 22:08:49 upgrade chromium-browser 25.0.1364.160-0ubuntu0.12.04.1 28.0.1500.52-0ubuntu1.12.04.2
/var/log/dpkg.log.1:2013-06-29 22:08:54 upgrade subversion 1.6.17dfsg-3ubuntu3 1.6.17dfsg-3ubuntu3.3
/var/log/dpkg.log.1:2013-06-29 22:08:55 upgrade libsvn1 1.6.17dfsg-3ubuntu3 1.6.17dfsg-3ubuntu3.3
/var/log/dpkg.log.1:2013-06-29 22:08:56 upgrade binutils 2.22-6ubuntu1 2.22-6ubuntu1.1
/var/log/dpkg.log.1:2013-06-29 22:08:57 upgrade gimp 2.8.4-2precise1~ppa 2.8.6-0precise1~ppa
/var/log/dpkg.log.1:2013-06-29 22:08:59 upgrade libgimp2.0 2.8.4-2precise1~ppa 2.8.6-0precise1~ppa
/var/log/dpkg.log.1:2013-06-29 22:09:00 upgrade gimp-data 2.8.4-2precise1~ppa 2.8.6-0precise1~ppa
/var/log/dpkg.log.1:2013-06-29 22:09:06 upgrade ufw 0.31.1-1 0.33-0ubuntu4
/var/log/dpkg.log.1:2013-06-29 22:09:08 upgrade gufw 12.04.1-0ubuntu1 13.10.0-0ubuntu2
/var/log/dpkg.log.1:2013-06-29 22:09:10 upgrade qbittorrent 3.0.x-0~4287-20130608~precise1 3.0.x-0~4290-20130629~precise1

Hm, nothing suspicious. Besides that you're using the PPA from Ubuntu Mozilla Security Team. (I don't get PPAs. Why would they upgrade chromium-browser and binutils?)

This thread advises it's a "Save Session" error (nevermind that it's more for Gnome and KDE):
http://ubuntuforums.org/showthread.p...1#post10896571

I was getting the same error message on Ubuntu (14.04) running XFCE:
Message - Invalid option: -session

In XFCE you can clear it easily in the GUI:
1. Settings
2. Session and Startup
3. Tab Session
4. Remove (Quit program) you don't want to startup automatically (e.g. spotify, skype)
5. Save session

Go to tab General, clear under Logout settings Automatically save session on logout
If you wish, you can explicitly select "Save session for future logins" when logging out, but beware if Spotify was running, you will get Invalid option message again.

So far I've only encountered this problem with Spotify.