Try BarTab - an addon that, when restarting Firefox, only loads the tabs you click on, as you click on them. No more having loads of tabs trying to reload at once! No more Firefox taking a gigabyte of RAM because you have rather a lot.. unless you click on them all.
The downside is that it only works with Firefox's native session restore and not with, say, Tab Mix Plus's one. But you can switch between the two to save sessions..
Thank you asim, for a comment in yendi for the pointer.