The main reason I have liked Firefox, is because it's "open-sourced". So anyone can create add-ons to customize the browser. Lots and lots of customizing can be done.
A big part of that attribute went away with Quantum, because the number of compatible add-ons dwindled hugely. Im sure eventually the old add-ons will be upgraded.
also, I have always had some issues with firefox and playing certain videos, or Flash and Java animations.
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