Functions well, does the job. A couple things would be great to make it better:
A keyboard shortcut to activate or deactivate it
An extension button to the right of the search bar
(both of those two would help prevent the cumbersome necissity to go into Add Ons menu to turn it on or off. I would love to switch between quickly)
Final potential addition would be a settings screen to choose to what size different scrollbars appear as.
Other than those desires, this does a great job

It works! (Firefox 71, Manjaro 18.1.5 KDE)
please add a switch :)

It works! I use Firefox on Ubuntu 18.04

Working well

Thank for this. Good works.

I like the idea but unfortunately this does not work with Firefox 68 on Ubuntu 16.04 64-bit.

Works as expected. Doesn't seem to break anything so far.