Resource Saver од Principle

No data collection, the plugin is entirely local on your device.

Resource Saver will save data on tabs as you unload them, then when you return to these tabs, it will immediately restore data to them, such as: Media/video time or input on a page.

To simplify the task of deciding which tabs to prioritize for unloading, this plugin also tracks all different tabs to evaluate their importance, such as how long they've been active for, if they're playing audio, etc.

The menu offers multiple buttons allowing you to unload some, most or all other pages, this lets you free up resources swiftly and efficiently whenever you're turning your focus to something else, like a game.

Then, whenever you return to the browser, you can either click back onto tabs and their data will be loaded immediately, or you can use the plugin to load some, most or all pages at once, data will be restored to all loaded tabs.

The amount of data stored on tabs is minimal, importance tracking process is minimal as well and only occurs once 90 seconds, making the plugin extremely light wight.

Resource Saver also has a Session manager tool, letting you save all your suspended tabs with their data to a session that can be restored later.

And finally, this plugin is solely local, and doesn't send data anywhere.

For the best experience:
Navigate to about:config, and set browser.tabs.fadeOutUnloadedTabs as true, this will gray icons if the tab is suspended, letting you see which tabs are suspended.
Pin the plugin to your toolbar for swifter usage.

TAB INFO:

By default, tabs will not be suspended if: They're whitelisted, they're playing audio, the website has an leave site warning, the tab is active or the tab is pinned.

LIMITATION:

It is impossible to hook the plugin into the suspension system itself, page data saving relies on the plugin suspending the page itself.
So, if the plugin isn't responsible for suspending a tab, that tab will not have any data to be restored.

The only cases where this would occur is either via firefox's automatic tab suspension or if you click a tab and click "suspend" instead of the "smart suspend".

Same-Origin Policy very often prevents the plugin from seeing or modifying the time of embedded video players, if the plugin does not remember the time of a video this is most likely the cause, there is no fix.