For me, this whole fiasco was the last straw: Quantum had already broken the plugins I use for vertical tabs, and then I found that the only reason Quantum was introduced was to allow this Looking Glass plugin to collect telemetry from the whole of the browser window, not just the pages being rendered. Like yourself, I've been a Firefox user since the Phoenix days, but this was a step too far.

I switched to Vivaldi, which is a Chromium with high levels of customization, including a vertical tab bar.