No, it wasn't. I was there.

At the time, Firefox's old engine was getting quite tired. It was single core, no sandboxing, a lot of cruft under the hood. Extensions had deep integration which would weigh it down. Load times were really rough, and things needed rewriting.

The Firefox you see now is part of the 2017 Quantum rebuild. that said, Quantum wasn't great either (because it jettisoned a LOT of great extensions), but it was necessary for survival. They did work on it and things got better over time, but there are still scars from that rug pull that was the extensions fiasco. I wish they had a better solution. RIP Tab Mix Plus.