I have ~350 tabs open across 12 thematically titled FF windows. I am not complaining about the # of open tabs - my complaint is that stock browsers suck and that I have to install a bunch of extensions from random sources to get to workflow happiness.

If you have more than 12 tabs open concurrently, vertical tabs are a hard requirement. There isn't even a workable vertical tabs solution for Chrome or Safari. Ironically, Edge and Orion have it out of the box. At least Mozilla has validated Sidebury, but they still make it a big pain to turn off the damn default horizontal tabs.