userContent.css doesn't fully load with e10s enabled

[lunakurame]

Firefox 57

@imports in userContent.css don't work when e10s is enabled: https://bugzilla.mozilla.org/show_bug.cgi?id=1416184. You can disable e10s by switching browser.tabs.remote.autostart and browser.tabs.remote.autostart.2 to false in about:config. Disabling it has side effects tho, the close button on the bookmark sidebar becomes a white rectangle and all icons look like if they were hovered all the time. More info in issue #14.

Firefox 59

It worked in alpha, still doesn't fully work in stable. @imports work in some places but not in others, eg. it works on Firefox's about: pages, but if you go to a non-existent address like https://shit.does.not.exist/ it still doesn't style them. Workaround: disable e10s, still causes side effects.

Firefox 60

Everything seems to work for now (60.0b8), whether e10s is enabled or not. Also disabling e10s doesn't have those side effects anymore. One new issue: opening a new tab (any page, not just about:newtab) sometimes flashes a white screen.

Firefox 61

The same as FF 60, except styles don't work on about:newtab. Hopefully it's just wrong selectors.