Most latest versions of Firefox (versions greater than 3.6) comes with the feature that their new tabs are opened in adjacent locations, just next to the present / working tab. And Similarly, when you hit on a link gets opened in its adjacent one.
But when we open some new tab by either visiting the File > New Tab or by hitting CTRL+T , then the new tabs gets appended at the right end side of the browser.
Well, this is a nice feature, because the adjacent related and working tabs improves our browsing experience. But if you want new tabs to be opened at the right-end corner as done by clicking the CTRL+T, you can do that as well. The procedure for implementing it is described below.
Go to the Firefox URL address bar, write about:config and hit the enter button. Please note this might void your firefox warranty. Now hit OK on button saying I’ll be careful, I promise.
Then visit to the Filter column text-box, and type browser.tabs.insertRelatedAfterCurrent, press Enter.
Now change browser.tabs.insertRelatedAfterCurrent value to false by double clicking on it. You can also toggle true or false by double-clicking the same.
Now exit from that tab, and you’re done with it. From now onwards, all new tabs will be opened at the right end side of your Mozilla Firefox Browser.