Have you allowed or blocked the plugin on Twitch.tv? I get the player interface but can't start playback if I block Flash on Twitch.tv. Otherwise I cannot reproduce this issue in 42.0 on Win7 64HP.

To check a page's permissions, right-click on the page and select View Page Info or follow these instructions: https://support.mozilla.org/en-US/kb/page-info-window-view-technical-details-about-page.

Permissions for twitch.tv are on or default. Nothing is turned off. I am using windows 10. But for instance, when there is a video ad on the page from youtube or whatever it will play but the twitch stream video will not play. I uninstalled and reinstalled mozilla, deleted my old profile and made a new one. Don't know what else to do so I am using chrome now since I can't fix this issue.

Alright, sounds messy.

Besides settings and plugins, cookies and cache are one thing that separates browsers and causes different behaviour. Do you have Firefox set to accept cookies (and also not to block cookies from Twitch.tv)? The setting is in about:preferences#privacy. Check the Exceptions, too.

You've done pretty much what Refresh does, but could you try that as well? https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings. This has helped a lot of people who had odd troubles with FF on Win10 after the upgrade.

Does the issue exist in a Refreshed Firefox with nothing but Flash installed (fresh installer dl'd from https://get.adobe.com/flashplayer/, no Theme, no Extensions, no other Plugins, no settings or profile imported, no Sync activated, yada yada), Enabled, and turned On for Twitch.tv? With cookies enabled?

I know Youtube uses HTML5 for playback, so their ads playing may not indicate Flash is working. I don't know how Twitch ads work, however.

One other thing that comes to mind is selective external interference, especially when it comes to inability to connect. What security software do you use?

Other than that, the solutions I am seeing have to do with following this process https://helpx.adobe.com/flash-player/kb/uninstall-flash-player-windows.html, disabling privacy add-ons, disabling Flash's protected mode http://kb.mozillazine.org/Flash#Disabling_Protected_Mode_in_Flash_11.3 (should NOT be necessary), switching DNS servers (mainly if you're on a restricted network like a campus or corporate connection), and finally disabling proxy/VPN (from about:preferences#advanced > Network tab > Settings button, select No proxy).