When composing a message and closing it without saving or sending, you get a dialog asking what to do (Save this message to your drafts folder (Drafts) and close the Write window?).

The dialog is missing the "Discard changes" label on one of its buttons and the button is empty, but still works.

The code is at https://searchfox.org/comm-central/source/mail/components/compose/content/MsgComposeCommands.js#4129.

I tested the call 'getComposeBundle().getString("discardButtonLabel")' does return '&Discard changes' properly. So either the dialog rendering got broken, or the arguments to Services.prompt.confirmEx() need some update.

This is most likely not related to TB but Firefox and this bug 1538983
As you can see more and more regressions are getting reported at the end, which they show the same issues we're having.

Should be from bug 1538983 yeah. Looks like a toolkit bug since we're just calling Services.prompt.confirmEx

This looks like a dupe of bug 1547346, can you try the patch from that bug? (that should hopefully land soon so alternatively you can wait for another Nightly build once it gets through)

Yes, https://hg.mozilla.org/integration/autoland/raw-rev/776c731d7c71 fixes the button for me.