I use the Auto Tab Discard extension, which turns tabs I haven't looked at recently back into the same ghosts as they are on startup. There's a bunch of configuration for how it chooses which tabs to discard. I'm away from my PC now, but I'm pretty sure it gives a right-click option on the tab to suspend one manually if you want.
I used this but had to stop using it as occasionally Firefox would require a restart for some reason or another and when in that state any paused tabs would lose their state if you clicked on them. Lost to much important state and it had to go.
Sidebery is primarily a horizontal (/tree style) tab lister, but this is also a feature of it (calls it 'unload' I think) on right click, really great if something's being a resource hog, but I might want it again later.
Auto Tab Discard works great for me as well. Would love to see something like this built-in, though.
I've also found cutting the content process limit in half to significantly improve performance. Combined with the above add-on my machine doesn't start swapping or become non-performant even with lots of sites open.
https://addons.mozilla.org/en-US/android/addon/auto-tab-disc...