I'm not aware of any Waterfox-specific issues or fixes, so this is some standard guidance that addresses the most common issues with the Flash Player plugin. I'm sure you've seen some of it before, but just in case:

(1) If you have any recorders/downloaders that interact with Flash media make sure they are as up-to-date as possible, or disable them temporarily.

(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 protected mode (Windows Vista and higher)

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

Note: the 32-bit and 64-bit plugins inhabit different folders, so you will need to do the mms.cfg fix in both folders to cover both Waterfox and Firefox.

Incidentally, if you have this problem on secure (HTTPS) pages and you notice a shield icon to the left of the URL in the address bar, check this article regarding the mixed content blocker: Mixed content blocking in Firefox

I had disabled hardware accel in Waterfox and it didn't fix the embeds, but doing so in Firefox fixed them for Waterfox! Odd. It didn't fix the constant Flash Player crashing, though. Thanks for solving the embeds!

For the severe issues, you really should disable the protected mode feature. If Windows gives you problems with permissions, try this way:

(1) In a My Computer or Windows Explorer window, open this folder:

C:\Windows\SysWOW64\Macromed\Flash

If that folder does not exist, then you are using 32-bit Windows, and you can open the following folder instead:

C:\Windows\System32\Macromed\Flash

(Note: This folder exists on both 32-bit and 64-bit Windows, but on 64-bit Windows Firefox uses the Flash player in the SysWOW64 folder instead.)

(2) Check for a file named mms.cfg:

Note: If you do not see file extensions such as .ocx, .dll, .exe, and .cfg on the files in this folder, then Windows may be hiding these from you. In order to work with file names accurately, it is best to unhide the extensions. This Microsoft support article has the steps: http://support.microsoft.com/kb/865219.

(A) If mms.cfg exists, drag it to your Documents folder where you can edit it without being bothered about administrator privileges

(B) If mms.cfg does not exist, open your Documents folder, right-click > New > Text File and name the new file mms.cfg

(3) Open mms.cfg from Documents into a text editor such as Notepad.

Add this on its own line (I put it last):

ProtectedMode=0

Save the file and close Notepad.

(4) Hold down the Ctrl key and drag the mms.cfg file back to the Flash folder to make a copy there, keeping the original in Documents

This change will take effect once the Flash plugin is completely unloaded, which may require exiting and restarting Firefox (allow a few minutes for all the Flash processes in memory to terminate after exiting Firefox).

I disabled Protected Mode, and Flash doesn't crash anymore on FF and I can use it normally. Embeds will work on FF with hardware accel enabled or disabled. They will work on Waterfox only with hardware accel disabled. Since disabling it slows down some elements on a website I use, I have reverted back to Firefox. Is there any issue in leaving Protected Mode forever disabled?

Hi Kelvets, my understanding of the benefit of the Protected Mode feature is that it isolates different Flash processes from one another which reduces the potential for a successful attack between Flash media from two different sites. Since most uses of Flash are noncritical (e.g., movies and games), I don't think this is important in day-to-day browsing. However, if you are using a Flash-based application to make purchases or submit confidential information, then you might want to close down other tabs using Flash until that is completed.

Interesting that when I installed Waterfox, Flash doesn't crash anymore in Firefox. It's still running the same Flash version. However, I have also disabled protected mode. One website, 7online.com , still shows only a very small footprint on the page for the Flash plugin element (in this case, videos). However, I was able to switch to full screen, and got the weather forecast (had to take a bit of a break, eh?!). YouTube and WhiteHouse.gov functioned normally.

Flash is installed on my computer both as 32-bit and 64-bit in separate branches of c:\Windows. I modified both mms.cfg files, backing up the original as mms.cfg.old before editing the mms.cfg file from another location (you're right, jscher2000; one needs to copy or move the file to a "user permitted" location; even though I am the Administrator on my computer, I can't open the file for editing from the containing folder). One should always back up an original file before editing it.

[I'm having some issues running both Firefox and Waterfox at the same time even though I've added the "-P" and "-no-remote" parameters to the command line in the start menu items. I'll have to find a way to have them not use the same profile — but that's a separate, but possibly related issue...?]

Now to test and see whether Waterfox is running Flash properly and whether it persists as a background process when/if Flash crashes. To that point:

Always make sure that there are no processes for WF, FF, and/or Flash before, during, and after making the changes that jscher2000 suggest (and which seem to work). Load Task Manager and check both the Processes tab and the Details tab. I had to restart my computer to get rid of about ten (yes, ten!!) "suspended" Flash processes which only showed up in the Details tab. I was unable to end these processes in the Details tab using End Task; however, restarting the computer did the trick. And I think it helped. (But I shouldn't have to do that... grrr...)

Waterfox runs fine with the above changes made. No Flash crash.

Jscher2000: Can the changes you suggest be edited and posted in a general help area of Mozilla so that more Win8.1 users can have easier access to it? I think there might be enough evidence that your suggestions work to put it out for wider user inquiry. If this is done, please let me know the URL so that I can pass it on.

Sent 7online.com (WABC-TV New York) screenshots of their malfunctioning pages. Since I can't replicate the 7online.com issues on other pages, this is not a Mozilla/Flash problem, in my estimation.

Generally, the problems seem to be with FF and WF running Win8.1 OS, fully updated (including Flash). Don't have issues with Win7 and WinXp running FF 28. Will not upgrade those machines to v.29, but may try WF on the Win7 computer — unless one of you expert commenter/moderators advise otherwise. Don't think it wise to run WF with Xp+64bit... right?

I consider the above a workaround until Flash and/or Mozilla can get the Flash plugin to work without changing default settings.

And the "fix one and the other works" thing is a mystery to me...

Hi MarkVictor, I'm sure there is a link to Adobe's page on protected mode somewhere in the Knowledge Base, but probably not detailed steps spelled out. I haven't reviewed all the Flash articles again.

I don't know whether Waterfox or other 64-bit executables can run on Windows XP. You should check with those sites on their system requirements.

In the long run, Mozilla is trying to eliminate the need for a Flash plugin by using a native player. You can read more about the Shumway project here: http://mozilla.github.io/shumway/