You configure in ~/.w3m/mouse as follows:

menu    "<= => R B"
button  1 0 1   PREV
button  1 3 4   NEXT
button  1 6 7   RELOAD
button  1 8 8   ADD_BOOKMARK
button  3 8 8   COMMAND "NEW_TAB; BOOKMARK; DELETE_PREVBUF"
button  3 0 4   SELECT_MENU
button  2 0 9   CLOSE_TAB

Then, w3m will display menu for mouse on top of line, next to tab.
Like this:

<= => R B                 [       Bookmarks        ][ 坂本浩則のホームページ ] x
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can open URL in new tab if you drag URL to the region of mouse menu.

~/.w3m/mouse syntax:

menu <menu string>

button  <num>  <left-column-pos> <right-column-pos> <func> [<arg>]
         :
         1 : left button
         2 : middle button
         3 : right button

So, above example means

menu    "<= => R B"
   # menu string is "<= => R B"
button  1 0 1   PREV
   # On "<=" (column 0 1), left button click, go to prev buffer
button  1 3 4   NEXT
   # On "=>" (column 3 4), left button click, go to next buffer
button  1 6 7   RELOAD
   # On "R " (column 6 7), left button click, reload
button  1 8 8   ADD_BOOKMARK
   # On "B" (column 8), left button click, add bookmark
button  3 8 8   COMMAND "NEW_TAB; BOOKMARK; DELETE_PREVBUF"
   # On "B" (column 8), right button click, open bookmark in new tab
button  3 0 4   SELECT_MENU
   # On "<= =>" (column 0 4), right button click, select menu
button  2 0 9   CLOSE_TAB
   # On "<= => R B" (column 0 9), middle button click, close tab