Firefox 3.6 puts newly opened tabs just to to the right of the current tab, instead of all the way to the right of all open tabs. I can’t really think of why it matters one way or the other.
Except, it really bugs me. For no good reason.
It’s not at all clear what you can do about this. I found out, though, and I’m noting it here. In the address bar, enter about:config, and click to accept any warnings you see. This gives you a list of very specific options you can adjust to change your browser’s behavior. Scroll down to browser.tabs.insertRelatedAfterCurrent and change it from true to false. Just double-clicking on the option’s name will make the change for you.
The warnings for this configuration page are a bit overblown. Every option that has been changed from the default is shown in bold, so if you make changes and don’t like how things came out, you could always go back and double-click on every bold option until they’ve all gone back to their defaults.