Everything else involving the internet, including updating other packages, works fine.

But updating Firefox always fails, with the following error:

Failed to fetch http://packages.linuxmint.com/pool/upstream/f/firefox/firefox_141.0.2%%2blinuxmint1%%2bxia_amd64.deb 404 Not Found [IP: 208.77.20.19 80]
Failed to fetch http://packages.linuxmint.com/pool/upstream/f/firefox/firefox-locale-en_141.0.2%%2blinuxmint1%%2bxia_amd64.deb 404 Not Found [IP: 208.77.20.19 80]

Any ideas how to fix this?

EDIT:

Turns out it was as simple as

sudo apt update

and then running Mint Update again. Now Firefox and whole system up to date.