He resigned April 3, 2014 after two weeks in the role.

According to https://www.reddit.com/r/firefox/comments/137ephs/firefoxs_d...

Google Chrome exceeded Firefox market share in early 2012 after a steady rise starting in 2009 afaict.

If his resignation was involved, it was a symptom and not a cause. The end was already forecasted at least two years earlier.