There is this addon called suspend tab which takes the right action but based on a wrong criteria, stopping (and unloading) all inactive tabs after a timeout. However, this feels like carpet bombing: I have tabs with books or articles I'm reading, and most of them are not offensive. I'd rather not have to reload them unnecessarily, as they are often quite big and take time to load.
Note this is different from a popular question about identifying offending tabs. I'm not interested in hunting such tabs manually.
I use Firefox and have no immediate intention to go for a different one. However, I welcome relevant answers for other browsers, because having a browser which goes easy on the CPU may convince me to switch.