In fact currently if I directly follow the steps to reproduce and switch to a workspace with the terminal in focus and Firefox doesn't, a single alt-tab works as expected and changes the focus to Firefox, so no bug.
If however I try to switch to a workspace where Firefox already has focus and alt-tab to another window (ie terminal or nautilus) it is impossible to switch away from Firefox with a single alt-tab, and I have to wait for the switcher window to appear and select the other window there.
I see this bug, but not for all applications.
In fact currently if I directly follow the steps to reproduce and switch to a workspace with the terminal in focus and Firefox doesn't, a single alt-tab works as expected and changes the focus to Firefox, so no bug.
If however I try to switch to a workspace where Firefox already has focus and alt-tab to another window (ie terminal or nautilus) it is impossible to switch away from Firefox with a single alt-tab, and I have to wait for the switcher window to appear and select the other window there.