User sheets are not applied to content pages in e10s mode. For example, if I add this chrome/userContent.css content to my profile: body { background: lime !important; } Running a stylo enabled firefox with "./mach run" shows about:home and https://google.com with no modifications. Running a stylo enabled firefox with "./mach run --disable-e10s" shows about:home and https://google.com with a lime background as expected. For chrome pages like about:newtab, user styles are always applied.

Over to Fernando for investigation.

nsDocumentViewer will take a UserContentSheet based on the backend type[1], but only one backend can initialize mUserContentSheet in content processes[2]. So user stylesheets will work only in one backend (currently, it happens to be the Gecko backend.) [1] https://dxr.mozilla.org/mozilla-central/rev/a748acbebbde373a88868dc02910fb2bc5e6a023/layout/base/nsDocumentViewer.cpp#2295,2302 [2] https://dxr.mozilla.org/mozilla-central/rev/a748acbebbde373a88868dc02910fb2bc5e6a023/layout/style/nsLayoutStylesheetCache.cpp#354-358

usercss.html passed with this patch and patches from bug 1355408. https://treeherder.mozilla.org/#/jobs?repo=try&revision=2404182b964e602641a615180f26eb9f39a73e55

Thank you Masatoshi! Since you already provided a patch I'm assigning you the issue.

Comment on attachment 8864314 [details] Bug 1358524 - Load userContent.css on both style backends in e10s. https://reviewboard.mozilla.org/r/135944/#review139318

Pushed by VYV03354@nifty.ne.jp: https://hg.mozilla.org/integration/autoland/rev/dcc5b9d4c114 Load userContent.css on both style backends in e10s. r=bholley