Hi, if using Customized Scrollbars then that is you problem. The userChrome no longer works in version 63 as there has been some code changes or maybe since is a beta that is not included in that release as it is a Beta. Or what ever you are using to customize them is not working.

If you are looking for help with that, sorry. Please return to the regular release channel for support.

Works for me with the current Nightly build although I had to make the screen wider. With some settings I even see both scroll bars. It is hard to see the scroll thumb as there is not much color difference.

Such custom sidebars are controlled via JavaScript, so if this isn't working then make sure that you do not block JavaScript.

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block (wanted) content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block (wanted) content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox shows the shield icon at the left end of the location/address bar in case content is blocked by Tracking Protection.

• click the shield icon for more detail and possibly disable the protection
• check the Web Console for more detail about what content is blocked by Tracking Protection
• "3-bar" menu button or Tools -> Web Developer

• https://support.mozilla.org/en-US/kb/tracking-protection-firefox

i disabled all ad&security extensions and disabled tracking, but not fixed. surely one of about:config items should become reset.

Can you locate the scroll bar in the Inspector like posted in the screenshot?

i fixed the problem. i reseted default options in "ScriptSafe" in its option, but problem not fixed, and then by disabling "Enable Right Click and Copy" extension problem fixed. Thank you my friends.