The old Firefox is plenty fast enough if you block the trackers that certain websites try to load up your browser with.
Any addons you have in your current Firefox won't work with Firefox Quantum -- including security-related addons.
There's a user review here
https://addons.mozilla.org/en-us/firefox/addon/firefox-quantum/#reviewsHe gives Firefox Quantum one star out of five (probably because zero stars wasn't an option) and says
NONE of the add-ons I used in Firefox are compatible with Quantum. 30% faster, NO FUNCTIONALITY.
Maybe there will eventually be similar addons for Firefox Quantum -- or maybe not.