Move the tab bar - Firefox

Use the ChromEdit extension to edit your userChrome.css file to add the code you need, as follows.

Move it to the Bottom

 /* Display the tabbar at the bottom */

 #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 */
   -mox-box-pack: start;
   -moz-box-align: start;
 }

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

 .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 James.