.... it will be good for you...
....trust us" ...
So...silly-old-me did so (as you do), and downloaded the new "Firefox Quantum Browser", overwriting my old Firefox Browser!
Now, my Old Firefox Browser contained all sorts of useful extensions and plug-ins, one of them being my lovely; "Foxclocks" program which displays the time zones around the World of all the places I visit in my Browser each day: Europe, North America, Australia and New Zealand, etc, in a handy strip along the bottom of the Browser!
Its always nice to know if my various Internet Chums around the World are wide awake in the daytime and listening to me, or fast asleep in bed at night and ignoring me!
...anyhoo...After installing the new; "Firefox Quantum Browser", I find that my beloved "Foxclocks" is no longer allowed, due to some strange hoo-haa called "Legacy", which disallows the installation of Foxclocks for some reason!
The new Quantum Browser also contained all sorts of confusing and unwanted "fancy stuff" as well...(A bit like Windows 10!).
So...I did an Internet search for some information on Firefox, and reverted my Firefox Browser back again to the previous familiar version which "allowed" all the useful bits and bobs again, including my "Foxclocks" program!
At the moment I am running Firefox version; "52.5.0" which is running as smooth as silk...with all my faithful add-ons installed!!
So there you go, Chums....Yet another case where; "newer" is not always "better"!
Paul...I like "Old Stuff"....a bit like me!....

P.S. I only use "Google Chrome" Browser for "casting" Youtube Videos from my Computer to my TV Screen...it works better/easier that way.