Normally, if you are on Page A and you open links to Pages B, C, and D without changing tabs, Firefox lines them up in that order moving to the right (A, B, C, D). This extension moves each new tab next to Page A as it is loading, so the order will be A, D, C, B.
Note: if you want new tabs _not_ opening using a link to be opened immediately adjacent to the current tab, there is a preference in about:config for that: browser.tabs.insertAfterCurrent (change to true). See:
https://support.mozilla.org/kb/about-config-editor-firefox
