То elle2:
Button "on/off" now there is! ))

To cubeleco:
I would like to have the option "Follow cursor Bottom".

While I was able to achieve only "Fixed Bottom":
bottomleft: position: absolute; top: 91%; color: white; background: #07ac; font: 15px arial black; border-radius: 15px; padding: 10px 10px 10px 10px; text-align: center; text-shadow: 2px 2px 2px #000; opacity: 0.9;

Can you help me?

Upd. 21/12/2017:

Need: a bar at the bottom of the window follows the horizontal position of the cursor.

So the panel will not overlap the controls web sites (usually located at the top or left) - how does the mode "Fixed".
The current mode of "Follow cursor" (when the panel is near the cursor) uncomfortable when using other add-ons (URL Tooltip WE, Link Alert) - they overlap each other.
Mode "Over image" is not convenient for small image - overlaps a significant part of the picture.
Mode "After image" is uncomfortable for bigger image - panel is often in the invisible part of the web page and needs to scroll down.
The mode "Before image" is not working fine when opened separately the big picture - the picture overlaps the panel, and centered the text is not visible. For test: https://img-fotki.yandex.ru/get/877700/9261163.19/0_9e522_278bb01b_orig
And at least on one site, this mode is buggy: http://nearspace.ru/ there are a few small pictures in a row and the bar starts to jump up and down.

So I want a versatile option. ))

Upd. 22/12/2017:

This is perfect! Thank you very much!!!

Now cosmetic remark: when I change the page zoom, the scale panel is also changing. It would be nice to fix it (if possible).

Upd. 23/12/2017:

Thanks again, cubeleco! I am fully satisfied. ))

Here's the final version (if someone is interested):

"Follow cursor"
bottomleft: position: absolute; top: auto; bottom: 2.5vh; color: White; background: CornflowerBlue; font: 2.2vh arial black; border-radius: 0.5vw; padding: 0.9vh 0.9vw 0.9vh 0.9vw; text-align: center; text-shadow: 0.15vw 0.3vh 0.15vw Black; opacity: 0.95;