One mitigating strategy you could adopt temporarily is to switch the Flash plugin to "click-to-play" so that sites can't use it without your approval. To do that, open the Add-ons page using either:

• Ctrl+Shift+a
• orange Firefox button (or Tools menu) > Add-ons

In the left column click Plugins, then find Shockwave Flash and set it to "Ask to Activate". On pages that want to use Flash, you will see some combination of the following:

• Link in a blank rectangle where Flash was blocked
• Notification icon in the address bar similar to a piece of Lego
• Infobar sliding down between the toolbar area and the top of the page

This is some standard guidance that addresses the most common reasons for Flash problems in Firefox. I'm sure you've seen some of it before, but just in case:

(1) Make sure all recorders/downloaders that interact with Flash media are as up-to-date as possible, or disable them.

(2) Disable hardware graphics acceleration in Firefox and in Flash

(A) In Firefox, un-check the box here and restart:

orange Firefox button (or Tools menu) > Options > Advanced > General > "Use hardware acceleration when available"

(B) In Flash, see this support article from Adobe: http://helpx.adobe.com/flash-player/kb/video-playback-issues.html#main_Solve_video_playback_issues

(3) Disable the protected mode feature (Windows Vista/7/8)

See this support article from Adobe under the heading "Last Resort": Adobe Forums: How do I troubleshoot Flash Player's protected mode for Firefox?