UPDATE: Smoothwheel’s developer took the time to write a thoughtful comment; please find it below the post.
I’ve spent so, so long trying to figure out how to get horizontal trackpad scrolling working in my installation of Firefox (on a Mac, running 10.3 – 10.5 while the problem persisted). I finally got it, and tracked it down to a problem extension: Smoothwheel.
For me, scrolling horizontally on the trackpad had scrolled the window vertically, for as long as I could remember. All the hints I saw indicated that a slight change to the preferences in about:config would resolve the issue, that the defaults in Firefox were for horizontal scrolling to move forward and backward in the history.
But those hints were from 2005, and I don’t think the forward/backward behavior has been the default for at least a year. So I started looking through my add-ons for potential problems.
The only one I had that should’ve been affecting scrolling behavior was Smoothwheel, which has options for compatibility mode and disabling. Sadly, neither fixed the problem. I tried disabling Smoothwheel at the Firefox level. No dice. Then I tried uninstalling it. Still no fix.
At this point, I thought, it couldn’t be Smoothwheel, so I went back to endless combinations of mousewheel and mousewheel.horizscroll settings in about:config. But nothing helped.
In the end, I took a wild shot and decided to delete [Profile Directory]/chrome/smoothwheel/ entirely to fix the problem. Very strange behavior for an add-on, residing in the chrome folder and not respecting disabling and uninstalling…
Based on the number of problems listed on Smoothwheel’s mozdev page, I can’t imagine I’m the only one having problems here.