Oh, there are kindred spirits here. I've recommended this on other tab-related discussions but Tab Stats is one of the few Firefox addons that have been worth using as a tab hoarder. Allows me to see which pages are open in multiple tabs and then deduplicate them if needed, or just looking at (or closing) open tabs based on domain.

(I'm not affiliated with the creator and it's not without its own annoyances, but still easily recommendable to anyone with thousands of tabs open)