On Ubuntu to move the the Firefox new tab button to the left rather than the right (replace the ‘me’ and ‘profilename’ bits below with the appropriate settings for your own system):
cd /home/me/.mozilla/firefox/profilename/chrome
cp userChrome-example.css userChrome.css
gedit userChrome.css
Add the following lines under the @namespace line:
/* Tab bar: buttons */
.tabs-newtab-button {-moz-box-ordinal-group: 1 !important}
.tabbrowser-arrowscrollbox {-moz-box-ordinal-group: 2 !important}
.tabs-container>stack {-moz-box-ordinal-group: 3 !important}
.tabs-closebutton {-moz-box-ordinal-group: 4 !important}
On Firefox 3.6 I needed to use the following instead of the block above:
tabbrowser-arrowscrollbox { -moz-box-ordinal-group: 2 !important; }
.tabs-newtab-button { -moz-box-ordinal-group: 1 !important; visibility:visible !important}
.tabs-container > stack { -moz-box-ordinal-group: 3 !important; }
.tabs-closebutton { -moz-box-ordinal-group: 4 !important; }
.tabbrowser-arrowscrollbox > .tabs-newtab-button{visibility:collapse !important}
/*button style for Past Modern*/
.tabs-container{padding-left:0px !important;}