I had been using TMP since I first discovered and fell in love with Firefox (pre-TMP days), back in 2004 at least. I loved Firefox already, and TMP made it so much more enjoyable... indispensable part of FF, in fact.
Amazing freedom of control over my tabs, the way they open, react, look, context menus, positioned, reload/close groups of tabs etc. The most important to me being multi-line tab rows which was lost when Quantum was released. I ignorantly used FF Quantum for about 8 months, stubbornly thinking TMP would easily update soon enough to be compatible, but after the 8 months I finally caved and searched for "What happened to TMP?", and saw it's just not compatible with FF Quantum. So with not much hesitation and with swift action, I uninstalled Quantum and went back to a "critically out-of-date" Firefox.
TMP is that good. Plus my other Extensions finally work again. 10+ years and running.