I use Tree Style Tabs.
It's a pity firefox still doesn't support it properly by allowing the top tabs bar to be hidden without userChrome.css hacking. I just ignore the top ones for now.
Regarding Tree-Style Tabs... I wish I could agree with you, I have been trying various Tree-Style Tab plugins on multiple occasions over the years, often for months at a time... before always giving up out of frustration and resorting back to tabs on top. I honestly can't tell you the exact reasons, but it's generally a frustrating experience... initial transition takes a couple days, plus issues like lack of proper hotkey support e.g. tab/ctrl-tab for tab switching, ctrl-f4 to close a tab, ctrl-t to open a new tab, some or all of those hotkeys tend to not work properly. Multi-screen support was also... lacking, for lack of a better description. There were various other minor annoyances and quirks with every plugin... and I tried various plugins for FF and Chrome before ditching all of them. I will probably try again soon, now that FF Quantum has been out for awhile, hopefully more plugins have been created.
As for tab-heavy users, I find FF Quantum to be horrible in this respect, and I keep missing Chrome, which allows a ton of tabs and resizes them, albeit slightly unreadable after ~40-70 tabs, but at least I can see the icon, which is usually enough to find what I'm looking for. FF resizes until ~5 characters of the title are still visible, thereafter requiring "< >" arrows to scroll through tabs... unfortunately if you click one of those arrows too fast (twice in < 1 second), it will skip 10-20 tabs or jump to the completely opposite end(!), which can be frustrating to get back to the tab that is just barely hidden off-screen. I have seen this complaint in forums but still haven't found a reliable workaround other than editing some javascript relating to the default theme (I forget the details)? I have reverted back to tab/ctrl-tab to cycle through tabs.