I use https://addons.mozilla.org/en-US/firefox/addon/panorama-tab-... to give me an overview. Currently at 232 tabs in _this_ window, my other windows have 445, 108, and 18 tabs.

Firefox does let you select multiple tabs and bookmark your selection as a folder, it also lets you expand a folder of bookmarks as tabs. I don't use either feature though, as my tab use tends to be more around not closing a tab until I'm done with the task associated with it, by which time I've opened another dozen and working out which I can close is too much effort.