I installed ff 55 from the mozilla repo. Scrolling in inactive window never
works. And sometimes it does not work even for active window.

It was always working with ff 52 and older.

Plasma 5.

This is an openSUSE "packaging" bug, apparently. It does not happen when I
download the ff 55.0.1 tarball, unpack it and run ff from there.

Safe mode does not help.

Upstream bug references -- there is another guy with the same problem on
openSUSE:
https://bugzilla.mozilla.org/show_bug.cgi?id=1359226
https://bugzilla.mozilla.org/show_bug.cgi?id=1390184

# rpm -q `rpmqpack |grep -i mozill|sort`
ca-certificates-mozilla-2.11-1.2.noarch
MozillaFirefox-55.0.1-1.1.x86_64
MozillaFirefox-branding-upstream-55.0.1-1.1.x86_64
MozillaFirefox-translations-common-55.0.1-1.1.x86_64
mozilla-nspr-4.15-1.2.x86_64
mozilla-nspr-32bit-4.15-1.2.x86_64
mozilla-nss-3.31-1.4.x86_64
mozilla-nss-certs-32bit-3.31-1.4.x86_64
mozilla-nss-32bit-3.31-1.4.x86_64
MozillaThunderbird-52.2.1-1.23.x86_64
MozillaThunderbird-translations-common-52.2.1-1.23.x86_64