With this mode switched on, W3M buffers are associated with the Frame on which they were created. See the code Lisp:w3m-fb.el.
Only tabs for the current frame’s W3M buffers are shown (with non-nil w3m-use-tab); other affected commands are ‘w3m-next-buffer’, ‘w3m-previous-buffer’, ‘w3m-select-buffer’ and ‘w3m-quit’.
Switch the mode on programmatically with:
(w3m-fb-mode 1)
or toggle interactively with M-x w3m-fb-mode RET.