For quite some time I have wondered why Iceweasel on my Debian Squeeze systems was slow. Symptoms were among the following.

• slow scrolling
• slow render times
• bad video playback
• slow tab switching

The problem is clearly related to the graphical rendering process. Worth mentioning is that on the same hardware the Chromium browser was snappy. After some trial, error and searching I found a single tweak that can be set to solve this issue! Iceweasel/Firefox utilizes XRender by default (see http://en.wikipedia.org/wiki/X_Rendering_Extension). XRender is a slow and often completely unaccelerated way of doing things. Configuring Iceweasel/Firefox to not utilize this extension will speed things up substantially. Recently a feature was added to the Mozilla branch to disable XRender (see https://bugzilla.mozilla.org/show_bug.cgi?id=743830). The setting for this is among the hidden configuration options. Open you browser and enter the following in the address field. about:config Acknowledge the warning message and promise me also that you will only change things here if you know what you a doing. At the top there is a search bar to search among the many many setting available. Go to the search bar and search for the following setting. gfx.xrender.enabled This is the setting that we want to change. Change this setting to False by double-clicking it in the list below. The line will turn bold to indicate that this setting is changed from the default value. Restart you browser and experience the snappiness. This tweak however may cause graphical glitches. There is a way to force hardware acceleartion and fix most of the glitches at the same time. If you do not experience any glithes you should not change this. layers.acceleration.force-enabled Setting this to True forces hardware acceleration (OpenGL) to be enabled. Keep in mind though that if you are using Xinerama you will only be able to use the web browser on the primary display. I actually ended up setting both of these tweaks with good result running Iceweasel 18.0.1 and NVIDIA 304.64.