This is a bug in upstream ("upstream" means, like, "original") Firefox. According to some sources this can be worked around by putting MOZ_USE_XINPUT2=1 to /etc/environment file, and rebooting (I tried exporting it directly in bash and running Firefox, but it didn't work for me, putting it to environment seems to work at least for the OP here).

For some reason I haven't found the upstream report about scrolling. I found though this report on launchpad that asks Ubuntu to package Firefox with MOZ_USE_XINPUT2=1 by default, which in turn has comments with links to some related upstream bugreports. FWIW, the reason they don't package it with the env. variable by default is summed up in one of comments by Nate Graham:

The problem is that setting the envar just substitutes some problems
with different ones:

• Without this envar set, there is no touchscreen scrolling, and touchpad scrolling goes three lines at a time rather than
  pixel-by-pixel




• With this envar set and a notification appears, touchpad scrolling is interrupted and mouse wheel scrolling stops working entirely until
  Firefox is de-focused and re-focused

The correct solution is to fix these bugs, not toggle between two
broken states