Ubuntu – Adobe-flashplugin package is virtual

I don't think forcing an insecure version of Flash is really the answer you want. Firefox is disabling it for a very good reason: Your alarm might not go off at all if somebody hacks your computer through Flash and encrypts your documents for ransom.

Why not install Alarm Clock or use something like Evolution's calendar and alarm functions? Both will work without Flash. Both will work without an internet connection... And there must be at least a hundred more alternatives. Keeping time is a very old problem.

As a side-note, you might want to submit a bug report to onlineclock.net to suggest they stop requiring Flash. You can play Alarm noises and videos without it and it's very easy to have them fall-back to something if Flash isn't installed.

adobe-flashplugin is available in the Canonical Partners repository, so you need to enable Canonical Partners.

P.S. It should be noted that the apps.ubuntu.com site is severely outdated. Canonical Partners currently provides Adobe Flash 28, not 10, and it's Adobe Flash 28 you'll install when you click that button.