Adobe announced that the current version of the Flash plugin for Linux is vulnerable and as a result, Mozilla has "soft-blocked" it. As a result, you have to activate it on a site-by-site basis.

According to today's bulletin:

"Adobe will provide an update for Flash Player for Linux during the week of July 12. The update will be available by visiting the Adobe Flash Player Download Center. Please continue to monitor the PSIRT blog for updates."

https://helpx.adobe.com/security/products/flash-player/apsb15-18.html

In other words, an update is on the way, but it isn't here just yet. It should eventually show up here:

https://www.adobe.com/products/flashplayer/distribution3.html

Meanwhile, if you are not accustomed to using the "Ask to Activate" feature for a plugin, here's what to expect:

When you visit a site that wants to use Flash, you should see a notification icon in the address bar and usually one of the following: a link in a black rectangle in the page or an infobar sliding down between the toolbar area and the page.

If you see a good reason to use Flash, and the site looks trustworthy, you can go ahead and click the Lego-like icon in the address bar to allow Flash. You can trust the site for the time being or permanently.

But some pages use Flash only for tracking or playing ads, so if you don't see an immediate need for Flash, feel free to ignore the notification! It will just sit there in case you want it later.

On Linux there is still no update for the Shockwave Flash plugin.

 Version: 11.2.202.481
 State: Enabled (STATE_VULNERABLE_NO_UPDATE)

• https://helpx.adobe.com/content/help/en/security/products/flash-player/apsb15-18.html
• Bug 1182751 - (CVE-2015-5122) Blocklist vulnerable versions of Flash Player plugin (18.0.0.203 and lower)