I frequently boot Arch Linux from a USB3 drive in various Intel based machines which I've discussed here before. Recently I was using the drive in a Dell e7240 which is a fairly nice, if older, Core i5 based ultrabook and although the drive was inserted in the USB port marked SS (USB3) performance in Firefox was awful. While performance in terminal windows seemed as quick as usual, typing anything in Firefox induced a delay that was so bad that the letters I typed into the URL bar or into a search form field wouldn't show up on screen until several seconds after I had typed them. Scrolling with the wheel on the wireless mouse would induce a pause every. single. time. I tried to scroll. It was actually quite maddening, and monitoring in htop didn't reveal any CPU or memory bottleneck. I disabled a bunch of extensions (plugins) that I wasn't using often anyway and restarted Firefox with not much change.

A quick Google search turned up this amazingly informative section on the Arch wiki. It turns out that OpenGL OMTC is indeed disabled by default when Firefox is running on this hardware. The page explains that this reduced performance setting might be getting set for a very good reason, but ignoring that good advice, I switched it on in about:config easily enough by toggling layers.acceleration.force-enabled true and restarting Firefox. Checking about:support again confirmed it was now enabled. Now Firefox is running just as fast as usual again! I will just have to use it for a while before I can really judge how stable the setup is, but at least I can use the machine.