I've been using Firefox continuously as my only browser since it was in beta and called Phoenix. This has never been even remotely true, at least on my experience.

To carry on with your anecdotal evidence, I'm going to disagree and say that yes, there was a time where firefox was just dog slow to open - on all 3 of the PCs I owned at the time, two using only 2-3 extensions and one using none. If there wasn't, I never would have switched to chrome.