Logical volume is not mounted but is in use. Please close all applications using this device (eg iscsi)

Increasing the size of a logical volume that's being used is indeed possible. That's one of the major advantages of using LVM in the first place.

It's just these gui tools ... meh ... forget about 'em.

1. (optional) Check how much space you've got left on your Volume Group with sudo vgdisplay

2. Find the name of your Logical Volume with sudo lvdisplay

3. Extend the volume by 5GB: sudo lvextend -L+5GB /dev/ssd/sysLV

4. Increase the filesystem size to match the LV: sudo resize2fs /dev/ssd/sysLV (ext4 only, see this link for other filesystems)

5. df -h should now report the correct size. Be happy, praise the command line!

As I can see Thunderbird running, you obviously missed an important step:

• Boot from a live CD.

What you're trying to do would be the real life equivalent of asking a surgeon to operate on himself, at home... ;-)

So prepare the operating room (boot the live CD so that the volume is not locked) and start operating, doctor! (follow the instructions in https://askubuntu.com/a/489909/45156)

You can try another GUI tool - KVPM. It helped in my case. I successfully extended a logical volume that was mounted as /home on the fly.

This seems to be a bug in the system-config-lvm tool.
Instead you can use the command lines:

lvextend -L +nnG /device 
resizet2fs /devic

This works even if the partition is the live /