Dead keys do not work well in Qt builds of Firefox or Fennec. This can be seen when using a Finnish keymap and trying to type characters such as ~ (tilde) or ü (u-umlaut). I would guess that other languages like Spanish are broken since letters like ñ (enye) can't be typed either.

In bug 1282866, I have removed the QT code from the Firefox tree with the approval of the active peer. That code is not currently maintained by any team. I have looked through the bugs in the Widget: Qt bugzilla component and I believe all of these are no longer relevant because the Qt code has been removed. If you believe that this bug is still valid, please move it to another more appropriate bugzilla component as you reopen it.