Move the tab bar - Firefox

Edit your userChrome.css file to add the code you need, as follows.

/* Display the tabbar at the bottom */

1. content > tabbox { -moz-box-direction: reverse; }

Move it to the Left/Right Sides

/* Display the tabbar on the left */
#content > tabbox {
  -moz-box-orient: horizontal;
}

.tabbrowser-strip {
  -moz-box-orient: vertical;
  /* note: you can set this to -moz-scrollbars-vertical instead,
     but then the scrollbar will *always* be visible.  this way
     there is never a scrollbar, so it behaves like the tab bar
     normally does */
  overflow: -moz-scrollbars-none;
}

.tabbrowser-tabs {
  -moz-box-orient: horizontal;
  min-width: 10ex;   /* you may want to increase this value */
  -moz-box-align: start;
}

.tabbrowser-tabs > hbox {
  -moz-box-orient: vertical;
  -moz-box-align: stretch;
}

.tabbrowser-tabs > hbox > tab {
  -moz-box-align: start;
  -moz-box-orient: horizontal;
}

/* remove the close-tab button */
.tabbrowser-tabs > stack {
  display: none;
}

If you want to move it to the right, add the following lines to the above:

/* Instead, display the tabbar on the right */
#content > tabbox { -moz-box-direction: reverse; }

Unabashedly stolen from clav.

Note that this does not work for Firefox 2.0b2 and newer.

Firefox 4.0 and above

The Tab bar will be at the top of the toolbars by default. To move the Tab bar back down below the Location bar use as in previous versions -- View (Alt+V) menu -> uncheck "Tabs on Top"