Firefox only: We've all been there — you open up 10 new tabs to read later, and Firefox crashes under the weight. Free extension BarTab waits to load newly opened tabs until you focus on them, so Firefox isn't overwhelmed.
Sure, it's nice when tabs load in the background, so when you're done with the page you're on you don't have to wait for the other tab to load — but at the same time, we all know how Firefox can be when too many tabs are opened at once (resource-hogging and crashy). If you're the type that opens lots of tabs so you can return to them later, BarTab will prevent them from loading until you actually focus on them. Sure, your browsing experience may seem a little slower, but your browser also won't crash so much — which, in the long run, is probably more efficient. Plus you get to keep the convenience of opening up pages in tabs to remember to read them later.
If there are certain sites that are slow loaders, or that you want to keep open in the background without focusing on them (like Gmail or Twitter), you can exclude them from BarTab's loading prevention in its preferences. You can also choose to restore tabs from your last session, as well as unload a tab after a certain period of time spent unvisited (so tabs that have been loaded but contain a lot of Flash, for example, won't continue to eat up RAM). If you find Firefox causing a lot of resource problems and crashes for you, this extension could definitely help ease that pain.
BarTab is a free download, works wherever Firefox does.