Firefox has a feature that unloads some tabs to save memory, but there is no way to configure it from the Firefox settings page. You can only turn it on or off in about:config, and there is no way to configure how it behaves.
There should be a setting in the Firefox settings page that allows you to turn tab unloading on or off without navigating to about:config (similar to the 'memory saver' feature on Google Chrome). Additionally, there should be options in settings to configure tab unloading, such as changing how aggressive it is, or changing the weightings of certain types of tabs (eg. making pinned tabs just as likely to be unloaded as normal tabs).
The option to change the weighting of tab types, in particular, would be very useful to me. I personally pin the tabs that I access less often, and leave the ones I access more often unpinned. Which means the tab unloading feature does the opposite of what I want, and I often have to unload my pinned tabs manually.