+++ This bug was initially created as a clone of Bug #1125956 +++ Bug 1125956 comment 11: OK, so I finally built Thunderbird and debugged this myself. The msg compose code uses a copy encoder <http://mxr.mozilla.org/comm-central/source/mailnews/compose/src/nsMsgComposeService.cpp#479> in order to encode the current selection, but it also requires the field to be encoded as text/html, which is not really supported by nsHTMLCopyEncoder. Before the patch to bug 116083, we would get lucky and never get into the two conditions which could cause mIsTextWidget to become true, but now we actually look at the styles. Now, plaintext emails are displayed inside a <pre> element, so the new code will detect them as preformatted and will encode the selection as plaintext forcefully, which confuses the msg compose code. I tried just using nsDocumentEncoder but that doesn't work either. Ideally someone would fix this brokenness in Thunderbird, but who knows how much work that's going to be. :/ For now, I'm going to make Thunderbird not use any of this logic. --- So this bug is to solve this properly.

One approach would be to introduce a new nsIDocumentEncoder flag for the behavior that mailnews expects, pass that flag from the mailnews side and inside Gecko's nsHTMLCopyEncoder, look for the flag instead of the current build time flag.