Both. It depends on how you specify what version of Firefox your add-on works with. And they do
change the format requirements and specifications sometimes. I messed with add-ons in earlier
versions and they quit working and I let them go because I didn't want to mess with learning the new
specs and such. It's a lot of work to learn and know the ins and outs of how all that stuff works
and keep up on it. But your mileage may vary. Good luck. (maybe it won't change much and/or for
awhile now, but I wouldn't count on it)