Having enjoyed using Firefox for years, and despite the memory issues that used to plague it, it still hung around on the desktop and has been used by a loyal crowd.
The key reason was the rich set of addons that allowed customization, the addons kept it in use despite some rather anonoying changes to cookie settings and other options. Basically the plugins allowed applying solutions to the declining functionality of the browsers settings.
However, the pending release of Firefox 57 in November 2017 will bring a stop for loading any extension that does not support WebExtensions. If the developers don't have the time to update their addons it means that it's time to move on.
While there are several well stated reasons for this, changes of this kind will have an impact on user preferences.
It seems the dominance of Google Chrome browser is about to jump to a new level given it supports a wide range of addons, that is if Firefox developers don't address the issue.