Found the solution! System Settings > Mouse & Touchpad > Slide the second (unlabelled) slider all the way left, and then one notch right.
Turns out that unlabelled slider is the wheel scroll speed. Setting it one notch from the left correctly reduces the wheel speed from 7.0 to 1.0 as Mir clients expect. ** Also affects: ubuntu-system-settings (Ubuntu) Importance: Undecided Status: New ** Summary changed: - Xmir Firefox scrolling is too fast (mouse wheel) when run under Unity8 (since Xmir 2:1.18.4-1ubuntu4) + Unity8 default mouse wheel speed is 7.0 (it should be 1.0) ** Changed in: ubuntu-system-settings (Ubuntu) Status: New => Confirmed -- You received this bug notification because you are a member of Ubuntu-X, which is subscribed to xorg-server in Ubuntu. https://bugs.launchpad.net/bugs/1607240 Title: Unity8 default mouse wheel speed is 7.0 (it should be 1.0) To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/qtmir/+bug/1607240/+subscriptions _______________________________________________ Mailing list: https://launchpad.net/~ubuntu-x-swat Post to : ubuntu-x-swat@lists.launchpad.net Unsubscribe : https://launchpad.net/~ubuntu-x-swat More help : https://help.launchpad.net/ListHelp