It was the extension signing that caused me to move on. I had written several small, but useful, extensions for my own personal use. I knew they were harmless, yet Firefox made it difficult for me to actually use them.

If I'm remembering this right, I think there was initially an about:config option for disabling the signature checks. But that was eventually removed from the stable releases. The workarounds were to waste my time getting the extensions signed, or to use some special unbranded build, or to use the Nightly or Developer Edition releases. None of those were acceptable to me. Then I learned about the planned WebExtensions changes, and knew it was time to move on.

I'm aware of the security-related reasons that were used to justify such changes. But for me they ended up taking away the main benefits that Firefox offered, namely being easy to extend, and giving me the freedom to use the browser as I see fit.