I have XP on one machine and 7 on the other. Firefox 8 works perfectly on XP but my other machine is a disaster. I can not access the add ons at all and everything is gone: too many tabs-roboform whatever?????????????????

Question

It seems that the upgrade has destroyed the entire firefox experience. What is also strange is that the desktop image is coming through the top of the Firefox page.

Answer 1

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Jon already touched on this, but to use your "new" network, without NAT, the subnet needs to be "known".
With NAT, you can place multiple hosts behind one (or several) IPs on your existing subnet.  I.e. you new subnet doesn't need to be known.  (In fact, you could use any subnet you want.)
The reason the Linksys worked well yet the 2600 did not, "Enterprise" class routers generally don't handle PAT as well many consumer class routers.  (NB: True NAT would probably work just fine, but what you need is to overload an IP, i.e. PAT).  So, many applications won't work.  (NB: I believe newer IOS versions do better, though.)
Many small Cisco Enterprise class routers, especially those from two generatations ago, were designed for "slow" WAN links, like single T1s or partial T1s, or perhaps for a few Mbps DSL.  If your link has more bandwidth the router may not have the performance to support it effectively.