Rated 4 out of 5 stars

Version 2.0 of this will plugin will not work 100% with FF4 (as others have stated). You can get it to work for loading however. Download the .xpi file and open it (its just a zip file). Edit the install.rdf and locate <em:maxVersion>3.6.*</em:maxVersion>, change the 3.6.* to, for example, 4.0. Save and rezip the file and rename from .zip to .xpi. Firefox will now accept the plugin.

What works:
When Firefox starts, the tabs are no longer loaded unless desired. Automatic unloading will, however, cause the loaded tab to remain loaded and a duplicate unloaded tab to be created. Changing to unloaded tabs, they will not automatically load, simply click the reload in Firefox. Note, when the plugin is updated, it will most likely not update if you make a change as described. Simply uninstall and install the updated whenever the author updates.

If you don't want to make this change, the plugin "Load Tabs Progressively" will work to slowdown Firefox loading all your tabs at once, and does work with FF4.