I updated to Firefox 11, & afterwards, my yahoomail & a few other sites began to load like a snail. What used to take a second or two on DSL began to take a minute or more, just to delete mail.
I believe what causes this is the keep alive for HTTP, & this worked for me, so it may work for you as well. If you have updated to Firefox 11, & you experience slowdowns like I did, try the following:
1) Open Firefox
2) Go to the address bar & type about:config & hit enter. Hit enter again when it comes up "I'll be careful, I promise!".
3) Type Keep in the search filter box & it will show 3 results. Double-click the one that says: network.http.keep-alive & change to false.
That's it! Your Y-mail & other sites should speed back up! Let me know if this helps.
Edit: I also went to HELP/Submit feedback in firefox menu & under Firefox made me sad, I suggested they switch HTTP Keep alive to False by default in newer versions, because it made my Y-mail run like a snail in FF11.
If enough people do this, perhaps they will change it!