Probably it's no longer supported by the author is a high probability. Also addons aren't just that addons and mozilla does own them. For changes in the firefox layout requires the use of "userChrome.css" edits to change the layout.

rparkins said

I want to change the look of the scrollbar in Firefox. Lots of web answers point me to a new scrollbar add-on, but the page https://addons.mozilla.org/firefox/addon/noiascrollbars/ does not exist. I'm using Firefox 60.4.0esr (64-bit) on OpenSUSE Leap 15.0

The NewScrollbars was a Legacy Extensions that worked on the old Firefox 56 and earlier versions. Since only current Firefox Release (64.0) and 60 ESR are currently supported Mozilla decided fairly recently to remove the Legacy Extensions from https://addons.mozilla.org/firefox/

Even if it was still listed it would not work on the Fx 60.4.0 ESR anyways.

Besides with userChrome.css you can change the scrollbar and colour and such in Firefox with the use of GTK3 themes on the OS.

Firefox 64+ supports some scrollbar properties if that is what you are referring to, so you are out of luck with Firefox 60 ESR. I don't think that there is already an extension that supports this feature.

• Bug 1460109 - [meta] Experimentally implement CSS Scrollbars Module Level 1

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)