Auto Tab Discard[1] does it automatically after a time. Tabs can be set to not get discarded, eg when form elements with changes are present, media is playing, tab is pinned, or just whitelisting.

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

You can simple exit and reopen your browser. Only the focus Tab will be loaded while all others are unloaded.