SDK never did this, possibly because default pref branch can't be "reset" according to [1]: > Note: This method can be called on either a default or user branch but, > in effect, always operates on the user branch. but we could maybe delete the branch, if the branch name was not changed using preferences-branch. [1] https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Reference/Interface/nsIPrefBranch#resetBranch%28%29

I don't really understand what the intent of deleting the branch on the add-on unload is, could you please elaborate ?

I think this should be a won't fix, it's useful to keep the prefs if/when the add-on is reinstalled.

this is about clearing *default* prefs added by the addon, not about prefs actually set by the *user*. old-school addons don't leave default prefs after they are uninstalled (after a restart). naturally, since we set them on addon startup, jetpack-set prefs are not persisted after a restart, but they are after addon disable/uninstall, which we can (and imho should) try to fix. also, this is probably related to bug 837023, though that one might also include interaction with user-set preferences branch.