The following comes to us courtesy of Simplehelp:
FORCE FIREFOX TO OPEN NEW TABS AT THE END
This tutorial will show you how to force Firefox to open new tabs at the end of the tab bar, rather than next to the existing open tab.
This new behavior (opening new tabs next to the current one) was introduced in Firefox 3.6. Some people like it, others don’t. If you fall into the “I want it back the way it was” camp – here’s how to change it.
In the Firefox address bar enter about:config and hit enter.
If this is your first time editing the Firefox settings this way, you’ll get a warning message. Just click the I’ll be careful, I promise! button.
In the Filter: bar, paste in the following:
You should see a Preference Name titled browser.tabs.insertRelatedAfterCurrent now listed
Note that it’s value is set to true.
Double-click on the word true. It should change to false.
That’s it! Close this about:config tab, and now when you click on links to open them in new tabs, the tab will be all the way at the end of your tab bar.