We already have Command+Shift+N to open the last closed window. Command+Shift+T (presumably for Tab) should not open a window. It should open the last closed tab in the active/current window. (And it used to do this, rather recently, unless my memory is really going.)
This behavior was changed in Firefox Version 116.0beta according to the release notes.
Since Firefox (unlike other browsers) uses Ctrl/Cmd+Shift+P to open windows in private mode, it has Ctrl/Cmd+Shift+N available to reopen closed windows.
While I initially found this behavior confusing after having switched to Firefox, I have grown to appreciate it more and more, since it allows easier separation of work and personal tabs by using multiple windows. (for instance, you can quickly reopen personal closed tabs from earlier, without having to first go through the 20 work-related tabs that you just closed after finishing a work session)
I understand, that not everyone will see it this way, and that there are different workflows and use cases, for which this change might have been an improvement.
Still, I would welcome a reversion of this change or an option to adjust this behavior in the settings or even just as a config option.
This thread hasn't gained much traction in the first ten days so I imagine the possibility of this being addressed are very slim, but this small quality of life improvement would be much appreciated.
Would it be helpful to submit the toggle suggestion as an "idea" instead of discussion? I simply rolled back to 115.0.3 when I found the change, but now with the new critical security vulnerability fixes announced (https://www.mozilla.org/en-US/security/advisories/mfsa2023-44/), that seems like a much less viable solution...
Edit: created an Idea.