Emilio, when running testsuite with the patch (locally) I'm getting this crash:

2:05.41 GECKO(53217) Assertion failure: sInServoTraversal || NS_IsMainThread(), at /home/komat/src/objdir-opt/dist/include/mozilla/ServoUtils.h:33
 2:05.41 GECKO(53217) #01: mozilla::ServoStyleSet::IsInServoTraversal() [/home/komat/src/objdir-opt/dist/include/mozilla/ServoStyleSet.h:103]
 2:05.41 GECKO(53217) #02: mozilla::Preferences::InitStaticMembers() [/home/komat/src/modules/libpref/Preferences.cpp:3464]
 2:05.41 GECKO(53217) #03: nsresult mozilla::Internals::GetPrefValue<int*&>(char const*, int*&, mozilla::PrefValueKind) [/home/komat/src/modules/libpref/Preferences.cpp:4439]
 2:05.41 GECKO(53217) #04: mozilla::Preferences::GetInt(char const*, int*, mozilla::PrefValueKind) [/home/komat/src/modules/libpref/Preferences.cpp:0]
 2:05.41 GECKO(53217) #05: nsXPLookAndFeel::GetFloatValue(mozilla::LookAndFeel::FloatID, float&) [/home/komat/src/widget/nsXPLookAndFeel.cpp:894]
 2:05.41 GECKO(53217) #06: nsWindow::GetTitlebarRadius() [/home/komat/src/widget/gtk/nsWindow.cpp:6111]
 2:05.41 GECKO(53217) #07: nsWindow::GetTitlebarRect() [/home/komat/src/widget/gtk/nsWindow.cpp:6438]
 2:05.41 GECKO(53217) #08: mozilla::widget::GtkCompositorWidget::GetTransparentRegion() [/home/komat/src/widget/gtk/GtkCompositorWidget.cpp:0]
 2:05.41 GECKO(53217) #09: mozilla::wr::RenderCompositorSWGL::AllocateMappedBuffer(mozilla::wr::Box2D<int, mozilla::wr::DevicePixel> const*, unsigned long) [/home/komat/src/gfx/webrender_bindings/RenderCompositorSWGL.cpp:149]
 2:05.41 GECKO(53217) #10: mozilla::wr::RenderCompositorSWGL::StartCompositing(mozilla::wr::ColorF, mozilla::wr::Box2D<int, mozilla::wr::DevicePixel> const*, unsigned long, mozilla::wr::Box2D<int, mozilla::wr::DevicePixel> const*, unsigned long) [/home/komat/src/gfx/webrender_bindings/RenderCompositorSWGL.cpp:185]
 2:05.41 GECKO(53217) #11: <webrender::compositor::sw_compositor::SwCompositor as webrender::composite::Compositor>::start_compositing [gfx/wr/webrender/src/compositor/sw_compositor.rs:1400]
 2:05.41 GECKO(53217) #12: webrender::renderer::Renderer::draw_frame [gfx/wr/webrender/src/renderer/mod.rs:4582]
 2:05.41 GECKO(53217) #13: webrender::renderer::Renderer::render_impl [gfx/wr/webrender/src/renderer/mod.rs:2009]
 2:05.41 GECKO(53217) #14: webrender::renderer::Renderer::render [gfx/wr/webrender/src/renderer/mod.rs:0]
 2:05.41 GECKO(53217) #15: wr_renderer_render [gfx/webrender_bindings/src/bindings.rs:623]
 2:05.41 GECKO(53217) #16: mozilla::wr::RendererOGL::UpdateAndRender(mozilla::Maybe<mozilla::gfx::IntSizeTyped<mozilla::gfx::UnknownUnits> > const&, mozilla::Maybe<mozilla::wr::ImageFormat> const&, mozilla::Maybe<mozilla::Range<unsigned char> > const&, bool*, mozilla::wr::RendererStats*) [/home/komat/src/gfx/webrender_bindings/RendererOGL.cpp:186]
 2:05.42 GECKO(53217) #17: mozilla::wr::RenderThread::UpdateAndRender(mozilla::wr::WrWindowId, mozilla::layers::BaseTransactionId<mozilla::VsyncIdType> const&, mozilla::TimeStamp const&, bool, mozilla::Maybe<mozilla::gfx::IntSizeTyped<mozilla::gfx::UnknownUnits> > const&, mozilla::Maybe<mozilla::wr::ImageFormat> const&, mozilla::Maybe<mozilla::Range<unsigned char> > const&, bool*) [/home/komat/src/gfx/webrender_bindings/RenderThread.cpp:0]
 2:05.42 GECKO(53217) [Parent 53217: Main Thread]: D/Widget GetScreenBounds [7fba6021a000] 95,55 -> 1280 x 1012, unscaled 95,55 -> 1280 x 1012
 2:05.42 GECKO(53217) #18: mozilla::wr::RenderThread::HandleFrameOneDoc(mozilla::wr::WrWindowId, bool) [/home/komat/src/gfx/webrender_bindings/RenderThread.cpp:355]
 2:05.42 GECKO(53217) [Parent 53217: Main Thread]: D/Widget Events sent from focus in event [7fba6021a000]
 2:05.42 GECKO(53217) #19: mozilla::detail::RunnableMethodImpl<mozilla::wr::RenderThread*, void (mozilla::wr::RenderThread::*)(mozilla::wr::WrWindowId, bool), true, (mozilla::RunnableKind)0, mozilla::wr::WrWindowId, bool>::Run() [/home/komat/src/objdir-opt/dist/include/nsThreadUtils.h:1203]

Any idea what can be wrong?