This article explains what to do when specific websites (not all) start to load, but never finish (i.e. the tab displays a spinning wheel and "Connecting..." for several minutes).
If none of the below solutions work, try the methods described in the Troubleshoot and diagnose Firefox problems article to see if they will address your problem.
A variant of the Vundo trojan is known to cause Firefox to have problems loading certain high-traffic sites, including Google, Yahoo, MySpace, Facebook, and more. Not all variants of the Vundo trojan can be detected or removed by malware scanners.
However, you should scan your computer for infections first. For detailed instructions, see Troubleshoot Firefox issues caused by malware.
On slow connections, too many HTTP connections may cause timeouts.
In the address bar, type about:config and press EnterReturn.