At least for me, I have ADHD—if it's out of sight (i.e. another bookmark in one of the hundreds of folders I have), it ceases to exist, no matter how important or interesting it is.
I have 6 virtual desktops for current projects, with all of the tabs (and other applications) for each project on a desktop, and with a Firefox window for each "topic" within that project. I go through and close out old windows periodically (i.e. when I need to free up desktops), saving anything I might want to refer back to with the Tab Stash plugin. Importantly, I also have the Auto-Discard Tabs plugin so they aren't using RAM until I need them again.
It might seem messy, but it's what works best for my brain. I do at least try to not have more than 12 tabs per window. On rare occasions I'm even successful! 🙃