To keep on-top of tabs in Firefox, I use 'Auto Tab Discard' [1] to discard tabs after a certain amount of inactivity. Then when I need to clean up my list of tabs, I click on any discarded tabs I want to keep, and then use my extension 'Close Discarded Tabs' [2] to clear the rest.

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

[2] https://addons.mozilla.org/en-US/firefox/addon/close-discard...