Can we validate that it is in fact losing market share first?
It's possible there are other factors such as the increased privacy capabilities and integrated tracker blocking and tendency for Firefox users to value and use these features and plugins, or even useragent string spoofing... I do the later so I look like a chrome/windows user because some sites will block features from Firefox for no reason (and also it's nice to be less trivially uniquely identifiable).
Where are the stats you are using and how are they collected?