Depending on the preferences, Firefox reopen all tabs at restart or shows a button to restore your last session with which you can reopen all tabs manually.. But only the active tab is really reloaded. For all other tabs only the titles and favicons are shown. These tabs are only reloaded if they are selected one by one. And again, only the selected tab is loaded.
This add-on will start reloading the neighboring tabs of the currently active tab immediately (after the currently selected tab has been loaded completely). So you can read the active tab and then switch to the next one without having to wait for it to be loaded.
The numbers of tabs to be preloaded left and/or right of the active tab can be selected in the add-on preferences.
