Joe, what I do may not work for everyone, but the first thing, if you haven't already done it, is to go to 'Privacy' in the Settings, go through the entire list and turn everything off using the little buttons. This will prevent a lot of unneccessary 'apps' running in the background, sending your data to all and sundry, and you can always turn something back on if you find you need it.
Once you have installed Firefox and Thunderbird and got them working to your satisfaction you can make them the default browser and email; they usually ask you if you want to do this.
I find it best to change one thing at a time so it's easier to trace the cause if there is a problem.
I have assumed you are using a conventional PC. Things will be different for a tablet or a touchscreen PC/Laptop.
Mike.