If 3DM2 suddenly stops working in your browser it is probably because your browser has recently been updated.
LSI (the company that bought 3ware) has information on this in this rather cryptically titled knowledge base article.
It seems from my experience that Firefox 11 and IE 9 are now both unable to access the 3DM2 web GUI.
[Update]: This is a fundamental problem with the SSL and TLS protocols. The browser updates fix the flaw, but the clients (i.e. 3DM) must be updated to be compatible.
There may be a fix described here in the Mozilla forums, but I have not investigated it yet. It seems LSI has updated the 3DM executable to be compatible, but you have to install it manually, which should hardly be a big deal.
[Update #2 (2015)]: LSI KB article describing fix.
I haven't done it yet, but it >seems< you do the following:
1. Stop the 3dm2 service (Windows) or daemon (Linux/FreeBSD).
Tip: /etc/init.d/tdm2 stop (to stop the 3DM2 daemon)
2. Copy the attached binary and overwrite the current binary.
Tip: 3dm2 lives in /usr/sbin
3. Restart the service/daemon (or reboot the system).
Tip: Command line: 3dm2
or /etc/init.d/tdm2 start (to start the 3DM2 daemon)
I haven't done it yet! (I'm too busy.) Use at your own risk!