Mr_Rubot.
Background color: no problem.
Instructions for image: tell me what you would write and i will use that.
Number of columns: add more columns or display less columns please explain.
Column wide spacer/divider: i don't know what that means please explain.

Sorry no support site, here is ok.

Edit:

Divider like this?
https://imgur.com/2TGi9M0

Edit 2:

Divider problem. It is too small to grab, drag or delete. Pick one here and tell me if you want an option to change color.
https://imgur.com/5S3R9x0

hr tag and css border style.

Edit 3:

Background color: done.
Divider: done.
Local image as button image: done (awkward for security reasons).
2-way toggle toolbar button: probably not for security reasons, reading Mozilla documentation.

Clean up and testing, new version in 3-4 days.

Edit 4:

That is strange it works for me. Lets try again.
Click browse button and select local image. Your image will appear under browse button. Right click image --> show image --> alt + left click button in sidebar.

Edit 5:

Just view image, alt + left click on button.
When you alt + left click on button, image in current tab is used as button image. The only thing in current tab must be one image you want to use, then alt + left click on button. Sorry i suck at instructions.

Edit: 6
I like your setup, looks great. I wish i could make 2-way toggle toolbar button or toggle on hover, but it can't be done with an addon. Mozilla says - To follow the principle of "no surprises", APIs like this can only be called from inside the handler for a user action. -

Edit: 7
I stopped trying when i read that line ^.
I guess i was wrong, i will try again.

Edit: 8
Toolbar button that toggles the sidebar: done.
V3 uploaded.

Edit: 9
Try this.
https://ufile.io/j75pl

Edit:
Creating a new add-on.
This is going to take a looooooooong time.
Check for new add-on next month.