Did you try to disable the plugin-container process as written in the http://kb.mozillazine.org/Plugin-container_and_out-of-process_plugins article ?

Yes, I changed dom.ipc.plugins.enabled.i386.flash player.plugin to false, but there was no change in behavior.

Unfortunately, I can't disable hardware acceleration because the popup menu you would normally get, isn't there. For example, when I go to a youtube video and control click on the black square where a video should be, I get the firefox control click menu instead.
The test page does not work, however both about:plugins and about:addons show that it is loaded and enabled.
---luminoth

Is seems as if the Flash plugin is not working at all for you in Firefox.

Does Flash work in Safari? If you have the same problem in Safari then the problem is with your Flash plugin installation. See Adobe's support document, Troubleshoot Flash Player | Mac OS

If Flash is working in Safari but not Firefox, then you should troubleshoot Firefox, for example, by disabling any Firefox extensions that may be blocking Flash in "Tools -> Add-ons -> Extensions" or by creating a new Firefox profile for testing purposes. See Troubleshoot and diagnose Firefox problems.

At this point, I think it may be better if you started a new topic (as a new question since this one was started by another Forefox user and is marked "solved". You can include this link for the background.

P.S. If Flash worked for you in Firefox 4 Beta 6 but not in Firefox 4 Beta 7 then the probem may be specific your system. Read the Release Notes to see what is different between Beta 6 and Beta 7, in case it helps:

http://www.mozilla.com/en-US/firefox/4.0b7/releasenotes/

If the issue is with Firefox 4 Beta 7 then I suggest, for the time being, that you uninstall it (e.g., drag the Firefox.app to the Trash) and reinstall either Firefox 3.6.xx from www.mozilla.com or Firefox 4 Beta 6 from http://releases.mozilla.org/pub/mozilla.org/firefox/releases/4.0b6/

Here is the download link for Firefox 4 Beta 6, English-US, for Mac users:
http://releases.mozilla.org/pub/mozilla.org/firefox/releases/4.0b6/mac/en-US/Firefox%204.0%20Beta%206.dmg

luminoth continued here: /questions/765367

Installed 4b7 a couple of hours ago and couldn't get to my cox.com home page. Just a white display with nothing in the bookmark bar, address bar, Nothing. Period. Have OS 10.6.5. Have current version of Flash. Disabled all three of my ad ons. Can't even get to Google. Whenever running a Beta version of Firefox, instead of installing it directly to APPs I put it on the desktop and named it BETA.app. Always worked over the years. Guess I'll have to trash this version.

Create a new profile exclusively for the 4.0 beta version and create a desktop shortcut with -P "profile" appended to the target to launch that profile.

• http://kb.mozillazine.org/Testing_pre-release_versions
• http://kb.mozillazine.org/Profile_Manager#Creating_a_new_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox