There are some compatibility issues between the latest Flash player and some Firefox add-ons or settings. Could you see whether anything in these articles helps:

• Adobe Flash plugin has crashed - Prevent it from happening again
• Flash 11.3 doesn't load video in Firefox

You might also consider disabling the new protected mode in Flash 11.3, since that feature is currently being debugged. See this support article from Adobe under the heading "Last Resort": Adobe Forums: How do I troubleshoot Flash Player's protected mode for Firefox?

I installed Flash 10.3 and it didn't help the issue

Flash processes are designed to be isolated in plugin-container.exe to avoid crashing Firefox.exe. When this happens, if you check the Processes tab, has plugin-container.exe launched as designed? If not, or if End Process on plugin-container.exe doesn't help, something else may be crashing the page.

If you open about:crashes are there any recent IDs related to this?

plugin-container.exe does launched as designed, I ended it's process and tried playing a video again via youtube and got the same result. It hangs up the entire browser, the only way I'm able to get the video to play is to end the firefox process via task manager. The browser never actually crashes therefore there isn't anything to report.

the only way I'm able to get the video to play is to end the firefox process via task manager

I'm confused by that. If you end the Firefox process, Firefox should close immediately and not play anything.

Could you try the new Reset feature? This duplicates certain key data from your active settings folder (AKA Firefox profile folder) into a new one, bypassing some add-ons and custom settings. Your plugins will still be active, but could be disabled manually if you like.

More information in this article: Refresh Firefox - reset add-ons and settings.

If the new settings folder has the same problem, you can switch back if you like using Firefox's Profile Manager. (This article describes how to start Firefox in the profile manager. I do not recommend removing anything until you are 110% certain that you will never need any of that profile's data in the future. Profile Manager - Create, remove or switch Firefox profiles)

Does it make any difference?

I'm confused why it's doing that also, which is the reason why I'm posting about the issue here. I've already reset my firefox and created a new profile. Still having the same issue