Disable message bar asking to allow a hidden plugin

Solution

I was annoyed too by the notification since it was introduced. There's no user setting which can control it, and probably there never will be. Disabling click-to-play wasn't an option for me, so I looked for possible solutions.

The manual way

I'm listing this first as it's the most efficient way.

Close Firefox.
Add the following code to your userChrome.css file:
```
notification[value="plugin-hidden"]
{
    display: none !important;
}
```
It is located in the chrome subfolder inside your Firefox profile folder. Create one in case it doesn't exists already. Here's an example: userChrome-example.css.

Note As reported by @Smylers, !important doesn't seem to be needed and could be omitted.
Start Firefox again.

Userstyle

If you have the Stylish add-on installed, you can get the Notification bar for 'hidden plugin' - disable style:

This style hide notification bar (infobar) for hidden plugin activations - eg. hidden frame or non-visible div.

Using an add-on

Install the Hide Plugin Notifications add-on:

Hides the hidden-plugin information bar in Firefox.

It basically injects the custom CSS code every time you visit a website. No restart required.

References

Firefox keeps on asking me to activate flash player
plugins.hide_infobar_for_blocked_plugin does not work
how to disable infobar for plugins (ask to activate)
userChrome.css

There is also a Firefox addon:

Hide Plugin Notifications:

"Firefox will now show an information bar for every site that uses a hidden plugin, to aid discoverability for users. This addon will let you turn off that infobar, if you know about the icon in the location bar. This is especially helpful if you have Flash marked as click-to-play."

It works like a charme!