Here's one for everyone, every now and again, general web browsing (facebook, etc), stops working. Browser just sits there trying to connect to the site. Now here's where it gets interesting. I set my browser up to use a proxy (in this case, a private one back at home, not on port 80), and voila! Browsing works again, turn this off, and browsing is again broke. Restarting the MiFi works for a coupla minutes, then returns to this. I know I'm nowhere near my data cap, and everything else (IRC, Skype, etc) work just fine.
This sounds like a new one. Perhaps your MiFi is having a temporary routing problem behind the scenes that the proxy is able to resolve.
Do you have the same problem with multiple browsers?
First thing I would suggest would be a level 3 reset to restore the default settings on the MiFi. Test it out in this configuration for a while and see if the problem re-occurs. You might also want to try reinstalling your browser /delete temp files if you are having the problem with a specific one.
Level 3 Reset