Assuming that you're using the latest version of Firefox (3.6.x), it splits Flash (and other plugins) into a separate process called "plugin-container.exe" that you can see in Task Manager separately from the main firefox.exe process, do you see that using a lot of resources when your Firefox freezes?
If not, then this may not directly be Flash freezing the browser.
(On another note, I have Flash and 32-bit Firefox running happily on 64-bit Windows 7 Enterprise and don't see these freezes)
Update in 2014
These days Mozilla have some knowledgebase pages on troubleshooting problems with the Flash plugin Adobe Flash plugin has crashed - Prevent it from happening again | Flash video won't play full screen on YouTube, Facebook and other websites. Though what fixed recent Flash not responding problems for me with Flash 13 on Windows 7 was Disabling Protected Mode in Flash 11.3.
The last resort option (that I followed) in Adobe's FAQ How do I troubleshoot Flash Player's protected mode for Firefox? is:
Finally, if these problems are blocking you from using Flash Player we recommend, as a last resort, you either revert back to Flash Player 11.2 or temporarily disable Protected Mode. We strongly recommend that Protected Mode is manually enabled once this issue is resolved.
To revert to a previous version of Flash Player, please see this FAQ: How do I revert to a previous version of Flash Player?
To disable Protected Mode, add the following line to your mms.cfg file located in:
Windows 32bit: C:\windows\system32\macromed\flash
Windows 64bit: C:\windows\syswow64\macromed\flash
If the mms.cfg file does not exist, create one using any standard text editor (e.g.. notepad)
Depending on your operating system, you might need to first save the mms.cfg file to a writable location (such as your documents or desktop folder) and then copy the file into the destination folder using Windows Explorer.
To re-enable Protected Mode, simply remove the line from the mms.cfg file.