Update as of 2023: This add-on is now depreciated.
With many tabs opened in Firefox for a prolonged period, it can consume a lot of memory.
Open Link In Silent Tab is a useful add-on that will open web pages in different tabs but not load them unless they are clicked upon. [This is similar to what happens when restoring previous sessions in Firefox where in tabs from before are visible on restore but not actually loaded unless switched to.]
Download and install it from here. No Firefox restart is needed.
Once installed, when opening a web page or a web link, a new option of “Open Link in Silent Tab” will be visible. Clicking on that will open the web page / web link as a tab in background but it will not load it unless it is clicked.
Another way to load tabs silently through this is by middle mouse click + Ctrl or by Ctrl+Alt+left click.
This is a handy add-on to prevent Firefox from becoming sluggish especially when there are a lot of tabs kept opened on a regular basis.