There is no Firefox 5.0, you were using Firefox 3.6.8 when you posted here. Probably an issue with one of your extensions, Rand McNally maps worked fine for me just yesterday.

Do you have that problem when running in the Firefox SafeMode?
http://support.mozilla.com/en-US/kb/Safe+Mode
Don't select anything right now, just use "Continue in SafeMode."

If not, see this:
http://support.mozilla.com/en-US/kb/troubleshooting+extensions+and+theme...

I find that I must disable Adblock Plus and (automatically then its twin is also is disabled) when I want to use Google maps or Earth. Once I am finished with maps, I re-enable it and then everything is just fine. Hope this helps. I have no idea what your usage is but this immediately came to my mind when I read your question. I am in no way other than an avid Firefox user-not anyone who helps with this site so judge accordingly. How did you find Firefox 5? I have all but beta and that will be 4-that one puzzles me but hope my fix helps you. Good luck

When I try to open Google maps, it reads that there is a problem loading... this may take some time...which really means that it will never open because I have tested it but when I disable that add-on, everything goes fine. I no longer use Mapquest since I tried Google maps but it may be similar.