On a brand-new Librem 15v4 running factory PureOS I can't install any addons in PureBrowser.

addons.mozilla.org doesn't seem to recognize the browser as Firefox, and prompts me to install Firefox instead of letting me install the addon. Since AMO is the only officially supported way to search for and install addons (searching in about:addons just forwards me to AMO), that means there's effectively no way to install addons.