I got 3 update notifications for firefox:
Yet when I click update, it fails with a "not found" error:
Any idea why this is? How to fix this?
Seems like you are failing to contact the US mirror so your update is failing.
Open a terminal or press Alt+F2 and type
You should be able to solve this by simply clicking the button to check for updates again. The package manager is looking for Firefox 9 but the binary is now version 10.
This happens when you automatically check for updates, the package version changes, and then you try to update on the stale information.
One you re-check, you will get the correct data & should be able to update with no issues.
If you are still having issues, try removing the firefox stable ppa.