Answers:
I would suggest NEVER use a beta program from yahoo. Beta money it is still contained by the carrying out tests phase. I don't know give or take a few you but I don't similar to human being the lab rat for yahoo. download a slightly elder edition of yahoo until the foreign beta is no longer contained by the carrying out tests phase.
use internet explorer, opus, netscape, or stop using yahoo beta
ditto to Jocco...
If you haven't tried IE7 even so, it's pretty angelic.
You also might want to report the problem to the folks at Mozilla. Perhaps here's a workaround.
Good luck!
One of your firefox plugins or add-ons is conflicting next to yahoo's ajax stuff...
Remove plug-ins and add-ons until you find the guilty one.
#