According to: https://wiki.cyanogenmod.org/w/ClockWorkMod_Instructions#Making_a_backup
ClockworkMod's backup/restore feature will generally only back up the
/system, /data, /cache, /sd-ext, and /dalvik-cache areas. Other
partitions such as /emmc, /sdcard, and /media won't be backed up.
In
fact, the backed up system will typically be saved to the /sdcard/
partition in the /clockworkmod/backup/ directory, which, depending on
your device, may or may not be actually located on a physical
removable SD card.
According to your question:
- The firmware?
Definitely Yes.
- My modifications to the firmware such as root, Xposed Framwork, installed Xposed modules, etc?
Definitely Yes.
- My installed apps?
Definitely Yes.
- App data?
Definitely Yes.
- Other data I had stored on the internal SD card?
Definitely NO, /sdcard is not backed up, because of there would be a CWM backup. Read: http://forum.xda-developers.com/galaxy-s3/help/how-to-backup-internal-sdcard-t1999853
Hope it help