Regardless of Add-on start-up speed, I found annoyance with Firefox itself. Occasionally on launch, I would be asked whether I wanted to install or skip updates to my Add-ons.

I found this process of having to choose to install or skip to be too much of a distraction, killing my flow when I was "in the zone" and wanted to look something up fast.

For this reason I stopped using Firefox and have been happy with Chrome ever since.