I had a LG phone running 5.1.1 and loved it's default browser. I upgraded to a LG phone running 7.0 and have not been able to find a browser that allows the same functions.
Correct that FF does not have any quick exit options.
If you have multiple tabs open, you can get quicker exits by going to the tab-list page, X-ing the individual tabs, then "back arrow" to exit the app.
I found that I could install addins, for FireFox, one that installs an "Exit" option and one that installs an option to "Close all Tabs". The "Close all Tabs" clears data too. There may be a way to keep it from clearing data but I have found it yet.
If you use Cheetah apps on your phone, don't blame anyone for not warning you about them. (Consider this a warning - some of their apps scream "we don't understand how Android is different from Windows".)