In case your Firefox profile has become corrupted, could you try accessing Flash media in a new, clean profile?

Exit Firefox completely, then on the start menu, search box, launch the profile manager this way:

firefox.exe -p

In the dialog, click Create and accept the defaults. You can call your new profile Fx15 to indicate that you created it in Firefox 15, or name it as you wish.

After starting up in the new profile, test your favorite Flash sites. Notice any difference?

You can exit Firefox and re-launch the profile manager to switch back to your normal profile. I don't recommend deleting any profiles unless you are 110% sure you won't need them again.

By the way, it would be useful if you could mention which settings you have changed. The headline suggestions for working around Flash problems usually include the following:

(1) Disable protected mode (Win Vista and Win 7 only)

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

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

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

orange Firefox button or classic 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) Check for and resolve conflicts with Firefox add-ons (e.g., using Safe Mode or a clean profile)

(4) For display issues with scrolling, disable Smooth Scrolling and Autoscrolling here:

orange Firefox button or classic Tools menu > Options > Advanced > General