A note for Ubuntu users; if Firefox is installed using `snap` (default) and you run `snap refresh` it will output "All snaps up to date" - but this is not true!
You have to close firefox, then run `snap refresh` for snap to upgrade firefox...
You don't actually need to stop it before running “snap refresh” though, it'll just be out of date as long as it is kept open. Once the application stops running, next time it is run the updated image will be used.
[caveat: I'm not a snap user myself currently, so my information may be inaccurate, take with a pinch of your favourite condiment]