First do yourself a favor unless you have a touch screen, download Classic Shell from here, it gives you back the good old Win 7 or Win XP interface and gets rid of the annoying Metro junk.
tools > options > advanced > general > “Use Thunderbird as the default mail application”
I know I am not the brightest bulb but my tools does not show up any where I can find like old windows XP. SSSSSooo where I can I find it in Windows 8??
Edit: oops I just looked and it seems thunderbird has changed since what I remembered. Do the same steps I said above but there will not be a place to check to make it default. Instead click “check now” button in the middle. Then click all the boxes.