Do you perchance have a cat?
An exceptionally large orange tabby is currently playing with my mouse... must be time for supper.
Seriously, if the device has not unexpectedly malfunctioned, the file that controls this behavior is probably named XF86Config
and it should have information about the pointer device or devices, namely mouse and/or trackpad.
Nothing that you could have done as an ordinary user, e.g.
in Firefox's configuration panel, could have (afaik) altered that file. Only root can twiddle with it. Sometimes you must kill the X
process, again as root, in order to make it pick-up changes, but you don't describe yourself as having done anything like that. All that you've said that you did was to change Firefox preferences, operating as an ordinary user.
You describe this as a "fresh"
9.3 installation. It .. worked okay 'before?' Place to begin, then, is "before what?"