User-Agent: Mozilla/5.0 (X11; U; Linux i686; de; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 Build Identifier: Mozilla/5.0 (X11; U; Linux i686; de; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 With some pages that contain a lot form-elements, like gmail's inbox page I get really low painting performance. Scrolling or resizing firefox is horrible slow. I was able to identify the use of GTK for widget-themeing as the problem, after disabling native theming gmail's inbox is rendered in 1/3 of the time it took before, which I validated using Trender: With native themeing disabled: Average: 26.807ms With native themeing enabled: Average: 72.078 As far as I know the problem is GTK requires you to render to X11Pixmaps as well as doesn't supply any alpha information, therefor the black/white trick. However wouldn't it be possible to render all form-elements to a large pixmap *once*, instead of doing many small readbacks with many context-switches, GPU stalls and cache-flushes? Reproducible: Always Steps to Reproduce: 1. Open a page which containts many form elements 2. Benchmark it with trender 3. set mozilla.widget.disable-native-theme;true 4. Benchmark again 5. Painting performance should improve dramatically.

Installing a different theme than the default GTK improved Firefox performance dramatically in my Ubuntu 15.10 installation with Firefox 42.0. I suspected the theme was affecting the performance as removing buttons from the toolbar made the web browser slower.

Also reported to <https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/990997>.

gmail's inbox opens up quickly for me with default theme on Ubuntu 16.04 with Nightly 51.0a1, 20160823030224