Ubuntu 16 and touchscreen scrolling in FireFox

This worked for me (firefox 57.0.4, Arch Linux):

1. open about:config in firefox to set dom.w3c_touch_events.enabled=1 (default is 2).

2. edit /etc/security/pam_env.conf and add MOZ_USE_XINPUT2 DEFAULT=1

3. reboot and restart firefox

(see also How can I get Firefox to recognize multitouch gestures on a touch screen? )

Found this solution elsewhere: if I start firefox from the command line with

env MOZ_USE_XINPUT2=1 firefox

then touchscreen scrolling works for me. To avoid having to start firefox from the command line, you can edit the Unity app launcher exec code to include this. Default .desktop resides in /usr/share/applications but we'll copy it over to our home folder so that the changes we make are immune to be overriden by the package manager on update.

cp /usr/share/applications/firefox.desktop ~/.local/share/applications/
gedit ~/.local/share/applications/firefox.desktop

find the Exec line in the [Desktop Entry] section and change it to

Exec=env MOZ_USE_XINPUT2=1 firefox %u

Change the other exec lines too, if you feel like it. Those affect the options in the right-click menu.

EDIT: At some point this stopped working (I think because of an extension I installed). To fix it, I had to go to about:config in the browser, right-click, add a new boolean browser.tabs.remote.force-enable set to true. Warning: this is not recommended and could break stuff.

I found a browser plugin called Grab and Drag which adds this functionality to FireFox.