Bug 486320 wants to remove the borders. With Lightning installed the borders are visible again because Lightning imports the global rules behind the messenger.css rule.
Created attachment 552057 [details] [diff] [review] Patch With border: none !important; it overrules the global rule. I removed the padding because this is already defined in the global rule.
Comment on attachment 552057 [details] [diff] [review] Patch I don't have a version of Thunderbird building on Linux, so I'm going to re-direct this to Andreas…
Comment on attachment 552057 [details] [diff] [review] Patch Seems to work as expected.
Something I noticed when running this is that the splitter between the mail and the calendar event sidebar looks a bit odd. Was there a bug about that already, or am I mixing that up with a Windows bug?
What do you mean with the splitter looks odd? Was this with bug 680657 applied. What's missing is the border on top of the status bar in Calendar and Tasks view. I'll file a bug about this soon.