I have had a problem with Windows Vista running the latest version of Firefox and with my XP laptop running latest firefox.
When sorting by due date - the entire computer freezes and is unresponsive in vista.
I can force quit the XP usually.
I have found that it is so slow in a browser that I just use it on my phone or use outlook tasks and sync - it is useless - especially when viewing a large amount of tasks (when you have 9 or so, no problem!)
Is this only happening in the latest Firefox?
Toodledo is aware of some issues with the latest build working slowly with Toodledo in some cases, and they are working on the issue. If that is what you are experiencing, in the meantime, you could try rolling back to a previous version of FF and see if that helps. You could alternatively try it in Chrome or Safari (I have a policy against recommending IE for any purpose).