If you are using the non automatic mode (where it scans about or available networks) choose yours then, select the type of encryption you are using then fill in the key, then select shared key. This may or may not work.
If this fails, then go to manual config. Select the wireless connection then properties.
Now deselect roaming, Enter the SSID. Set the mode to DHCP so your router deals with all the IP, DNS and stuff like that.
Now this is the important bit. Select an encryption type that you are NOT using and make up a key. Then click ok. A box saying configuring will come up.
When it is finished, click properties then put in the correct encryption and key. Then click ok.
It will do the configuring again.
When it is done, execute Firefox, and see if you get a connection.
This method has worked for me on 6.10, 7.04 and beta 7.10. Automatic now works for me but hopefully you can get some luck with this.
Good luck