The Ctrl+Tab is really a Windows feature - it isn't programmed into MUSHclient as such. It usually switches back to the last window you had open, so if you had (1) and then (2), then Ctrl+Tab goes back to (1) and not (3).
Probably the easy thing, to go between specific worlds, it to use Ctrl+1, Ctrl+2 etc.
The intention is that if you have the little activity button bar open (with the coloured numbers) and you seen one change colour (eg. 5) then you just type Ctrl+5 - thus going direct to that world.
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