Vertical scroll bars have been hijacked - firefox v89.0 (64 bit)

[Bret Busby]

Hello.

I am running the above version of firefox, on UbuntuMATE Linux v 21.04 .

I have recently noticed that the vertical scroll bars in firefox have
been hijacked, and replaced with a designed to be unusable, useless,
thin, thread-like system of things.

I get the same, thin, vertical scroll bar in also, Brave, Vivaldi, and
Chromium web browsers.

I get the more traditional width vertical scroll bar, in Seamonkey,
Konqueror, and Thunderbird v78.8.1 (64 bit), although, whilst the
Konqueror vertical scroll bar is the desired width, it is not the
expected colour.

In Seamonkey v2.53.7.1 Build identifier: 20210410163459 the vertical
scroll bars are as they should be - usable, and, consistent with the
system windows theme, but, in firefox, the vertical scroll bars are
designed to be unusable, and, are not consistent with the system
windows theme.

Does anyone know how to regain usable vertical scroll bars in firefox,
or, do we need to uninstall it, and, install a sufficiently earlier,
insecure, version of firefox, in order to regain usability?

Thank you in anticipation.

-- 
Bret Busby
Armadale
West Australia
(UTC+0800)
..............