Cannot install firefox on Windows 7

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I was having trouble with the Firefox v25 updating. I have Avast anti-virus installed and running Windows 7. Since I could not install update, I uninstall Firefox. Then I tried download the stub and the whole package for v26. Every time I try to install, it started up and extract the files, I see 100% extracted and then nothing happened.

I even disabled the anti-virus software but it made no difference.

Please help.

Thanks!

I didn't find the right solution from the Internet.

References support.mozilla.org/en-US/questions/981481

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