To start, please download and run the file at http://people.mozilla.org/~akeybl/Firefox_10_Diagnosis.exe . It will extract a folder with all the testing builds of Firefox.
Please try to open the respective firefox.exe -- sometimes listed as firefox (Application) in each of the folders (numbered 0 - 16). For example inside the 0 folder, double-click firefox0.exe. For each one, wait a minute or so to see if it starts or if it crashes and then fill out the questions below.