Harry Bloomfield wrote:

Each time Firefox downloads one of its automatic updates, until the
update is installed, browsing in Firefox comes almost to a painful
standstill.

firefox upgrades are generally only about 10 seconds to download and 10
seconds to upgrade and restart the browser here

I notice similar issue with Win10 updates, once they are downloaded
and ready to install.

Those can take longer, but during the download phase it doesn't hammer
anything ...