Hello,
That sounds like an issue with the storage. Sometimes during Firefox update the storage gets broken and addon (or addons) needs to reinstalled. Make sure to restart Firefox after uninstalling (before installing it back).
Also, please visit this storage test page to see if the main database engine in your Firefox is working as it should:
https://firefox-storage-test.glitch.me/

Best regards,
Juraj Mäsiar

Hello,
Could you write me to my support email more details?
Mostly, what browser and operating system are you using?
If you are using Linux or Mac, you should get a permission popup when you enable right click scrolling in the settings and you need to approve it.
Best regards,
Juraj Mäsiar

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