I'm having lots of trouble with 4.0b7, I've been using this beta since the beginning and things seem to have gotten a lot worse with 4.0b7.
I'm running it on a 2009 MacBook Pro 13" OS X 10.6.5
The first thing I've noticed is the Menu at the top (Fire, Edit, View, etc) will become unresponsive at some point when using the browser. This has happened a few times and the only way I could quit was to Right Click the icon in the Dock and quit that way. The browser itself remains usable, I can click web links and type in addresses... it's just the top bar that becomes unusable. Quitting was the only way to correct this.
I have a new iMac (Sept 2010) running OS X 10.6.5 and haven't noticed any problems with Firefox 4 Beta 7 menu items, although I haven't been testing Beta 7 very long. (I have Firefox 4 Beta 7 installed in a separate Applications: "Firefox4Beta" folder and still have Firefox 3.6 installed, by the way. I'm using separate Firefox profiles for each.)
If Firefox 4 Beta 6 worked but not in Firefox 4 Beta 7 then the problem may be specific your system. Read the Release Notes to see what is different between Beta 6 and Beta 7, in case it helps:
Important: This forum is for Firefox support, not to report problems to Firefox developers. Make sure you use the Feedback button in Firefox 4 Beta to let Firefox developers know about the problem. If you need more space, you can use http://hendrix.mozilla.org/ but be sure to include "Firefox 4 Beta 7" in your feedback report.
I have a Mac running 10.6.5 and have been having this unresponsive problem with firefox for the longest time. Several versions have dones this and not firefox updates have resolved the issue. Firefox will not respond to key commands nor will it respond to the mouse and selecting menus. But it will let me select file and then quit.
This is really annoying happens either if I leave my computer asleep or just on for hours or overnight then the next morning nothing I have to quit and relaunch firefox for it to respond.
You would likely get a better answer if you started a new thread (ask a new question) in the Firefox Support forum. Even though your issue is similar (Mac OS X 10.6, Firefox not responding to key commands or mouse clicks selecting menu items), starting a new thread will let others trying to help see details about your system and Firefox version that may help solve your problem.