For months now, I’ve found Firefox on my Linux laptop to sometimes be sluggish and a CPU hogging, particularly when scrolling. T-Mobile UK and Engadget were the worst affected. Visiting t-mobile.co.uk saturated the CPU for several seconds whilst rendering. The result looked horrible – grainy, and badly pixelated.

I’d attributed this to X, Nvidia, browser sniffing, Flash and Javascript/CSS. Of course it was me all along. Firefox 3 has a feature called Full Page Zoom, it doesn’t just resize text, it scales everything on the page. I had zoomed these pages, then forgotten.

If any of this sounds familiar, try reseting your zoom level:

  1. Visit the page that scrolls slowly or looks pixelated.
  2. Either press Ctrl + 0, or click Edit → Zoom → Reset.
  3. If text is now too small to read, enable Edit → Zoom → Zoom Text Only, then zoom in with Ctrl + +.

Firefox should now scroll the site smoothly and quickly. The zoom level is remembered on a per site basis, so repeat this for any other pages affected. If you would like to control zoom from the toolbar, try the PageZoom extension. If you would like to set the zoom globally, try No Squint (courtesy of AncientPC on Al-Osaimi Techlog).

The question remains why Full Page Zoom can be so sluggish, and under what circumstances. Also, why does Try Firefox 3 full page zoom on Mozillalinks performs so poorly for me, regardless of page zoom.