Firefox

If you have that many tabs open you are doing something seriously wrong

No, not really. This is rather common. The line might be blurry with tabs unloading, but bookmarks and tabs are still different things. And don't just reject people's workflows - that's how you end up with chrome.

I cannot even imagine a scenario that would warrant this many tabs

One common scenario is shopping for parts for complex systems, like cars for example. One might have a dozen tabs open for parts themselves, a tab or two each for specifications, a dozen tabs per part for listings in different shops, each with a few tabs looking for ways to deliver the thing. It blows up to 100+ really quickly. And you really need them all open because you need to jump back and forth comparing and cross-checking all of them. And then if you haven't managed to get everything done in one session, bookmarking them and re-opening again later takes considerable amount time, so you're better off just opening a new window and keeping it all in the background until you return. At least that's how I've seen people do it.