Firefox tries to open unknown type files which leads to crash

Firefox tries to open unknown type files which leads to crash I just started using portable firefox 3.6.8 when I wanted to download some files, the firefox was trying to open the file instead ask me what to do or save it. There is no such file type under "Tools-Options-application". So I can't change its' action . It seem that my firefox's default action is trying to open it. Looks like it's different from others. How can I let firefox ask me what to do when it runs into a unknown type file? (with a different extended file name for example filename.xxx ). ps. I have used firefox 2.0 for more than 3 years. and I never encounter that problem.