"Create a sticky (…) Ctrl + Shift + C" is the combination used in Mozilla Firefox for Developer Tools.

Text in predefined fixed colors is hard to read in most cases:

Foreground: #FFFFFF
Background: #5AA885
Contrast ratio: 2.848: 1
1.4.3 Contrast (Minimum) (AA)
Fail for large and regular text
1.4.6 Contrast (Enhanced) (AAA)
Fail for large and regular text

Foreground: #FFFFFF
Background: #4DD35C
Contrast ratio: 1.947: 1
1.4.3 Contrast (Minimum) (AA)
Fail for large and regular text
1.4.6 Contrast (Enhanced) (AAA)
Fail for large and regular text

Foreground: #763A37
Background: #FF5D54
Contrast ratio: 2.858: 1
1.4.3 Contrast (Minimum) (AA)
Fail for large and regular text
1.4.6 Contrast (Enhanced) (AAA)
Fail for large and regular text

Foreground: #FBDCF4
Background: #F235C7
Contrast ratio: 2.725: 1
1.4.3 Contrast (Minimum) (AA)
Fail for large and regular text
1.4.6 Contrast (Enhanced) (AAA)
Fail for large and regular text

Foreground: #EEF5FC
Background: #2689DE
Contrast ratio: 3.342: 1
1.4.3 Contrast (Minimum) (AA)
Pass for large text only. Fail for regular text
1.4.6 Contrast (Enhanced) (AAA)
Fail for large and regular text

Foreground: #76502E
Background: #FE963C
Contrast ratio: 3.26: 1
1.4.3 Contrast (Minimum) (AA)
Pass for large text only. Fail for regular text
1.4.6 Contrast (Enhanced) (AAA)
Fail for large and regular text

There is no way to use the extension to set a color flag for web pages via the extension button indicator color. That is, it is enough for me to see that I previously set the current page to "red color" or "green color", without adding any elements to the page itself.

Worked well initially but I think the author periodically wipes out stored notes on his server. Not good if so. I've moved over to OurStickys which allows sync with Google accounts.

I really wish I could have rated this higher, as it is exactly what I needed except that it wasn't reliable. You cold not rely on a stickie sticking to a web page. You could return to that very page five seconds after having placed a stickie on it and it would not be there. Even if you hit hide/show stickies repeatedly. I relied on these stickies for short notes such as shipping costs or price per item reminders on products I was considering. It was disappointing that these stickies would not be there on the page when I returned.

the irony is that this addon doesn't work in https://addons.mozilla.org/. I wanted to use it to remember if something didn't work.
didn't tried it in a big number of websites, but it seem to work ok outside of this place.
ctrl+shift+c opens developer console for me.
I'm using manjaro linux 18 / firefox 68.0.1
forget what i said, this doesn't work, it is bugged and doesn't let me remove it from my system

Works on 1 Mac but not on another. The Sidebar is empty. (Reinstall doesn´t ake any difference)

AddOn not reliable

How is one to use this with android without a mouse?
I have tried many note add-ons and this is the only one that installs on android firefox, BUT I can't get the damn notes to show up on a page?
I dont see any "sidebar" like you mentioned.
Please inform me how to use this. Do I need to create custom gestures or something?