I suggest you use the release build instead of a developer build if minor bugs are of such import to you you want to change mail clients. Daily does not come with a guarantee that it will even start. Thunderbird has a release version, a beta, an alpha (earlybird) and a developer build (Nightly). While nightly builds tend to be fairly stable, I have had ones that simply crashed over and over, had no write button and a myriad of other odd regressions from changes As this is a testing bug, have you looked for entries in Bugzilla and filed a bug if you can find nothing relevant? see https://bugzilla.mozilla.org/