My Firefox has them left aligned automatically. It may be a "side effect" of an extension I have, but I don't think so.
Anyway, I was able to get the stylish script to download.
It really was a simple script.
If you just write a new script and copy and paste the following startign with "@namespace" and give it a title it should fix it. I already edited it.
@Scott, I wasn't trying to hijack the topic, I know how userstyles.org has been having trouble lately so I figured I would post this while I actually got the script up.
/* ---------- Tabs Text Centered - [http://userstyles.org/styles/5455] ----------*/
text-align: left !important;