For Firefox there is the 'Auto Tab Discard' addon. It has the ability to discard (unload) tabs after a certain amount of time or the number of non-active tabs is greater than a number, these are set in the addons options. Also has the ability to whitelist tabs, web sites, so that they never get unloaded.

https://addons.mozilla.org/en-GB/firefox/addon/auto-tab-disc...