Andy Burns explained :
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 ...

I don't think Harry means the time taken to do the actual download or
update, but just in general terms.

For instance, I hardly ever power my main computer down, just letting
it sleep overnight. I may reboot it every couple of weeks or so.
However, if browsing starts to slow down to the point where it annoys
me, I can always guarantee that a Firefox update has downloaded and is
waiting for me to close the browser so it can action said update.

It's a repeatable situation that happens every time there's an update.