Make Active Tab Wider
Makes the active tab wider than the other tabs

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Tweak Type:	Display |Tabs
Fx 2:	YES
Fx 3:	YES
Fx 3.1:	YES

Simple tweak to make your active tab wider. Using ChromEdit add the following lines to your userChrome.css file:

/* Make the active tab wider */
#content tab[selected="true"] {
min-width: 200px !important;
}

Once you have added these lines, click Save, then Restart; your active tab will be wider than the inactive tabs when Firefox restarts. Notes: You can substitute any value for the ‘200px’. This tweak works for Fx 2 & 3.

Source