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 clav.