I've been playing around with a beta build of Firefox's 3.6 browser for some time, and while it's been completely stable, its new tab behaviour has annoyed me.
When opening a link in a new tab, by default the tab will appear next to or near the tab in which the link was selected. Think Internet Explorer or Chrome tab behaviour.
Since IE, Chrome and now Firefox do this, it'll probably become de facto for browsers, but if you wish to remain a new-tab Luddite like me and restore Firefox's previous tab behaviour (where all new tabs appear on the far right of the tab bar), follow the following steps:
Type into the address bar about:config and hit enter.
The about:config page should look something like this.
Once there, enter into the filter browser.tabs.insertRelatedAfterCurrent
When you see the browser.tabs.insertRelatedAfterCurrent entry, double click on it to toggle the entry from true to false.
And you're done. Enjoy your new browser experience of knowing where to find the tabs you opened.