Script will warn you at specific times left for your hit. The times can be set in the options menu.
Same situation with Google Chrome, at least with regards to the initial voice announcement not staying off.
The voice at beginning option will work in a future update. I'll check what's going on with firefox to see if it can get fixed. I usually just use chrome.
couple of bugs
On Firefox, color warning does not work. Also, time remaining voice alert comes on at the start of ever HIT, even after I've disabled it.