question about firefox portable app download?

i need to put it on my sisters pc she has so many viruses and spyware her internet explorer wont open anymore so i need to install the firefox onto her pc or can i install it onto my flash drive which is better to do if i install firefox on my flash drive can i plug it in go on and be able to download antivirus onto her pc ? should i save the download onto my flash drive or should i install it onto my flash drive ?

Install on USB.

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