GUIMiner does not connect on Slush's pool I checked my firewall, and I tried the solution in Bitcoin question GUIMiner won't start mining on Slush's pool.

It is still saying connecting forever.
Without seeing exactly what you are doing on GUIminer, this is just a guess because it is a very common problem beginners run into.

When GUIminer was last updated, Slush was using difference credentials to mine on his pool.

These are the current credentials to use as of this post's date, per Slush's home page:


  http://api.bitcoin.cz:8332
  (Main pool URL) 
  
  or
  stratum.bitcoin.cz:3333
  (If you have Stratum-compatible miner)


In GUIminer you are probably using the built in defaults for Slush's pool



Notice that the Website field does not match one of the two current URL's from Slush's home page.

You will have to change the server drop down to other and then manually enter the correct credentials.

This is for the main server:



This is for the Stratum server:



Try the Stratum server first. Stratum protocol is more efficient and lessens the load on the pool. If it doesn't work then switch to the main server.



I'm sure you already know, GPU mining is no longer profitable (hasn't been for quite a while). You will likely only make about a penny or two per day worth of Bitcoin while your power cost will probably be about one to two dollars per day. I'm betting that you are just getting started and trying to learn how to do it first before trying anything else, like purchasing an ASIC. That's good. I did the same thing.