it is also possible that the dll firefox is attempting to use is not one that was installed by firefox... often programs use dlls that are part of windows... it is then possible that you have a dll that has become corrupted.
First do as the others said... go to control panel and remove software... remove firefox.... then reinstall firefox...
if that doesn't solve the problem then I would guess that a dll is corrupted in one of the windows folders... If the error messages tell you which dll it is trying to inject when it dies you know which one to look for... if not your next step is to open the windows folder... and windows/system and windows/system32... sort the files by detail and date... if you have one dll that has a very recent date then that is likely one that has been changed and is causing the problems you need to replace that file with a good one... if the corruption was more random then you would most likely have to reinstall windows because it can take a long time trying to figure out what file is messed up and windows has hundreds that it uses.