Sometimes when loading pages in firefox I get a server not found error. If I refresh the page then it loads. This doesn't always happen but it happens enough that it gets annoying. How can this be fixed?
This is a very hard one to answer - It can really be a number of issues.
As you said it is "Server not found", I am inclined to believe this could be issues relating to DNS timeouts.
The problem could lie anywhere:
Well you get the picture.
You only have direct control over 1. so double check that the connection is sound. As it appears that the problem is not limited to one or two sites (though you don't say this explicitly) that would tend to rule out 4.
However, as the problem could be either physical or software it's difficult to offer any firm suggestions. See if you can get your ISP to test your connection - there might be an issue there. You could also try rebooting your router - you never know it might clear the problem (at least for a while).