Hello,
Yes, Firefox is blocking all addons in the PDF viewer since Firefox 60.
There is some security issue, more info here:
https://bugzilla.mozilla.org/show_bug.cgi?id=1454760
Best regards,
Juraj Mäsiar

Hello,
I'm not sure what you are looking for but if you want to customize scrollbar width, you can't. Firefox supports only 3 scrollbar widths - none, small and default.
Best regards,
Juraj Mäsiar

Hello,
I've created an issue for the double-click action:
https://github.com/fastaddons/ScrollAnywhere/issues/40
I will look into it for the next release.
Have a nice day :)
Best regards,
Juraj Mäsiar

Hello,
Check the Options page - there you can set what button should be used for scrolling and you can test there how it works (in the right part).
Best regards,
Juraj Mäsiar

Hello Sofia,
This addon is meant to be used with a mouse. So if you use touchpad on your Mac, then it's not gonna be useful.
If you do have mouse, make sure to visit Options page where you can setup what mouse button is used for scrolling and where you can test the scrolling (in the right area).
If you have more questions, you can contact me directly on my support email.
Best regards,
Juraj Mäsiar