Firefox 4 does not support flash.
Firefox 4 supports it just fine. The fact it's a beta means a lot of plugins and extensions will not support it. When the final is released a lot of extensions will get updated so they work in Firefox 4. However, not every extension developer supports his or her extension so some extension will cease to work (and exist in the end). You can't blame Firefox for that though.
Try disabling the out of process plugin support in about:config. Simply search for dom.ipc.plugins and set all of those from true to false. This will do magic for Flash and crashing Firefoxes.
For those who experience problems: use the feedback button and/or submit a bug so the developers will be informed about the bug you encountered. That's the only way to have them fix the bugs in time so we users end up with a stable and fast Firefox 4.0. If themes/extensions don't work bug the person who creates the theme/extension, Mozilla can't help you with that.
One piece of advise for the OP: if you want stuff to work properly stop using things like beta versions! By using a beta version you are accepting and agreeing (it often is a part of the license agreement) to the fact that stuff might not work properly and you may loose data. Mozilla clearly warns users about this. Ignoring warnings like those is your own mistake, not theirs or anyone else's.