Some keyboard shortcuts (e.g. Ctrl-Shift-T in Firefox or Terminal) are broken when using Dvorak layouts

When using Dvorak based keyboard layouts in Ubuntu 14.04, some keyboard shortcuts appear to be broken. This is the case for Ctrl-Shift-T, which I sometimes use in Firefox to open a previously closed tab, or in gnome-terminal to open a new tab. It doesn't work when the English (Dvorak) or another Dvorak based layout is in use, but it starts working again as soon as I switch to English (US) or another QWERTY based layout.

I could use the 'xev' program to confirm that the keys pressed produce a different result when the Dvorak layout is in use.

Any help with this issue would be much appreciated.