"server not found"

[#1]

I'm new to the apple scene, had my iMac for about a month now. For the last 3 or 4 days when I start the iMac and try to go on the web via firefox I get the message 'server not found" I then follow the steps below and this seems to solve the issue. Any suggestions / help on how I can resolve this issue?
look forward to your replies
many thanks
Craig

[#2]

Are you ticking the 'remember password in keychain' box? I suspect not, which is why it's asking for your wireless key each time. If you remember the password, it won't need to do that
Don't worry by the way, the keychain is secure and safe to store your passwords in.

[#3]

Follow Matt's suggestion first
If that does not work, you might want to look at this link
http://osxdaily.com/2012/02/22/mac-wont-remember-a-wireless-network-password-heres-how-to-fix-it/
You might also want to look in System Preferences - Network, to check the wi-fi configuration manually.

[#4]

Thanks guys...........
tried Matt's suggestion, when i started up this morning I got the same symptoms as yesterday only this time I didn't have to enter the wi-fi key. It did it's check and my connection was up and running.
going to try your suggestion the morning msssltd......
many thanks to you both really appreciate you both taking the time to answer.........
Craig

[#5]

Tried your suggestion msssltd, on restart it all went well, my homepage loaded up no bother. Then without warning the web connection failed and I got this message (see attached)
went through system prefs - network - assist me - diagnostics it did its checks again and I'm connected.
going to try another restart and see what happens......
kind regards
Craig

[#6]

Bit of a hunch....
Where is your iMac located relative to the router?
It looks like the iMac is struggling to negotiate encryption. 
There are several possibilities why that might but with a Mac, the most likely is signal issues (distance and/or interference).

[#7]

Which router are you using?