if you use the add on https://addons.mozilla.org/en-US/fir...orter/?src=api
then your add ons will continue to work, in fact only two of mine ceased to work. of around 45 addons
add it before you upgrade if you choose, the reason for upgrading generally is to combat security issues
also plenty of add ons have been updated.
when you start to upgrade it checks your add ons and tells you which wont work, then you can cancel the upgrade, however as i said before if you have the compatibility checker installed then there will be very few it will say wont work when you try the upgrade.
another way to go is to install mozbackup and save your profile and settings etc before you upgrade, then you can roll back if there is a problem and restore your profile