> Why is that?

Because Firefox's "powerful extension model" basically constituted direct access to its internals, which prevented the Firefox developers from making the architectural improvements that were so sorely needed to make it performant.

> Firefox is slow with many tabs open on Win10, Ryzen X3900 with 32gb of RAM (writing this from FF)

Admittedly I'm on Linux, but I currently have approximately 7800 tabs open (no, that is not a typo). Firefox is a little laggy in its UI, but Chrome has never even managed to have more than a few hundred open without becoming unusable, so I'd say that Firefox is doing fine here.

Interestingly, the slowdown actually seems to originate from rendering the tab bar, not from the tabs themselves.