Thanks for the suggestion!
I thought about "locking" tabs also, as you can see at: http://trac.mucommander.com/wiki/TabSupport
but I thought that the effect of the locking will be different: when you lock a tab its location won't change until it's unlocked, and when trying to enter any subfolder/archive file on a locked tab, it will be opened in a new tab
I think that the way you descibed will be confusing as the user might forget that he locked the tab, and then he'll be suprised that the tab's location changed from its previous location (at best he'll notice that and will change it back, in the worst case he'll make operation like copying or moving to a different destination than planned)
Anyway, I'll discuss it with Maxence