Over in bug 1450753, I'm planning to remove xul style overlays. Looks like there's two uses in thunderbird: https://searchfox.org/comm-central/search?q=style&case=false&regexp=false&path=*.manifest

There are a few more actually: https://searchfox.org/comm-central/search?q=style&path=jar.mn For Thunderbird add-ons this will be fixed by bug 1448808, but we'll need to load manually for Thunderbird proper.

So the replacement is to load the css files directly in the xul files where possible, or insert them via JS, e.g. like contentStylesheet = aDocument.createProcessingInstruction("xml-stylesheet", 'href="chrome://messenger/*.css" type="text/css"'); document.insertBefore(contentStylesheet, document.documentElement);

Richard, can you take a look at these, please. https://searchfox.org/comm-central/search?q=%25+style&case=false&regexp=false&path=jar.mn ignoring calendar/ and dead im/ only yields 8 of these: % style chrome://messenger/content/customizeToolbar.xul chrome://messenger/... That's assuming bug 1450218 has landed (which it will RSN(TM)).

This removes the style overlays and adds them directly to customizeToolbar.xul.

Comment on attachment 8972687 [details] [diff] [review] styleOverlay.patch Looks straight forward and seems to work. Thanks.

Pushed by mozilla@jorgk.com: https://hg.mozilla.org/comm-central/rev/3a08130870ef Remove XUL style overlays in Thunderbird. r=jorgk