How did Firefox get to be such a terrible web browser?

The web browser market used to be competitive. But Google’s Chrome has taken over. It seems like many software and internet companies are more interested in mobile apps than desktop web browsers today.
I used to use Microsoft’s Internet Explorer before that company basically gave up on the web browser market. IE became so crash-prone that I had to switch to another browser.
My preferred browser became Firefox because I’m a big user of bookmarks and Firefox has a better system for viewing and managing bookmarks than main alternative Google Chrome.
But Firefox, at least on Windows 7 Professional, is terrible. It locks up, becomes inoperable or crashes so frequently that I have to keep Windows Task Manager open to force close it when it’s stuck. Firefox is so awful that it crashes multiple times per day. (I’m currently using version 55.0.1.) Firefox especially doesn’t like multiple tabs being open and running any pages with video or Flash.
I now keep Google Chrome open to do important tasks online. I use Firefox mostly for viewing news. (I’ve downloaded the Opera web browser, but haven’t really started using it yet.)
I don’t have a lot of hope that Mozilla can fix its Firefox browser at this point. But fingers crossed.