Purebrowser supports installation of addons, and promotes in several ways the Mozilla addons service, which features both free and non-free add-ons.
Previous releases of PureBrowser (releases based on Debian Iceweasel) linked at some places to the FSF addon web page https://www.gnu.org/software/gnuzilla/addons.html
PureBrowser should not promote external external addons, to avoid implcitly promoting non-free code, and to avoid leaking user data to third-parties.