If you don’t want to interrupt the current loading tab, at the same time want to open many tabs in the background, here is a useful add-on for Firefox.
The add-on called Load Tabs Progressively lets you load the background tabs one after the other. It will limit the number of concurrent loading tabs and/or unread loaded tabs, so that you can browse without any problems.
You can set any number of tabs to be loaded at the same time (depending on your connection speed and requirement), maximum number of unread loaded tabs, next tab to reload etc. with this add-on.
Download this add-on (currently beta) at Mozilla add-on site.