Scrolling has recently become incredibly slow. Firefox is not my primary browser on this computer, so I can't tell exactly when the problem was introduced or whether it coincides with installation of other software. It looks as if hardware acceleration was no longer available. Both scrolling and page loading are very slow, Firefox appears to "lag" for a second or more. CPU usage spikes to near 100% on one core while scrolling in Firefox, vs. a mere ~25% on Opera for the same webpage. While scrolling, the mouse cursor also lags: instead of a single, smooth movement, it jumps around the browser, but the effect stops as soon as the mouse cursor leaves the Firefox window. I have no such problems in current releases of Opera, Chrome or IE8. What I've tried so far: - Uninstall Firefox, checking "Delete my preferences" - Reinstall - Disable all plugins (currently re-enabled after Reset) - Troubleshoot -> Reset Firefox None of these did anything, so I followed the advice from http://kb.mozillazine.org/Scrolling_is_slow Most interestingly, the problem completely disappears when I lower the hardware acceleration settings for the graphics adapter, see screenshot: http://www.yurgon.de/uploads/misc/ff21_hw_accelleration_wtf.png However, in FF advanced options, the checkbox "Use hardware acceleration when available" has no effect on the problem. With "Use smooth scrolling" checked, the problem is more pronounced, with this option unchecked it is less pronounced, but still very annoying. It's quite possible that the problem appeared after a recent windows update (if so, it probably would have been after the April 2013 patchday, and it definitely appeared before installation of the patches of the May patchday this week).